[Fuchsia] only download fuchsia deps when necessary (#51439)

This is a quick follow up of #51072 to only download fuchsia deps (via download_fuchsia_deps gclient arg) when necessary. Considering the pr/51072 is very huge, keeping these condition changes in a separate change would be cleaner.

The wiki has been updated already since setting the download_fuchsia_deps gclient arg explicitly won't break the existing workflow. I also mentioned in the wiki that building and running fuchsia needed a linux box (an explicit assertion in the with_envs.py).

Bug: http://b/40935282

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

.ci.yaml

@@ -209,6 +209,13 @@ targets:
     properties:
       release_build: "true"
       config_name: linux_fuchsia
+      # pm in fuchsia-sdk is required when uploading artifacts, so we have to
+      # download the fuchsia-deps in root builder.
+      gclient_variables: >-
+        {
+          "download_android_deps": false,
+          "download_fuchsia_deps": true
+        }
     # Do not remove(https://github.com/flutter/flutter/issues/144644)
     # Scheduler will fail to get the platform
     drone_dimensions:
@@ -304,6 +311,11 @@ targets:
       add_recipes_cq: "true"
       config_name: linux_license
       clobber: "true"
+      # Ensure licenses in fuchsia deps can be correctly processed.
+      gclient_variables: >-
+        {
+          "download_fuchsia_deps": true
+        }
 
   - name: Linux linux_web_engine
     recipe: engine_v2/engine_v2

DEPS

@@ -110,9 +110,7 @@ vars = {
   # Checkout Fuchsia dependencies only on Linux. This is the umbrella flag which
   # controls the behavior of all fuchsia related flags. I.e. any fuchsia related
   # logic or condition may not work if this flag is False.
-  # TODO(zijiehe): Make this condition more strict to only download fuchsia
-  # dependencies when necessary: b/40935282
-  'download_fuchsia_deps': 'host_os == "linux"',
+  'download_fuchsia_deps': False,
   # Downloads the fuchsia SDK as listed in fuchsia_sdk_path var. This variable
   # is currently only used for the Fuchsia LSC process and is not intended for
   # local development.
@@ -1210,7 +1208,7 @@ hooks = [
   {
     'name': 'Download Fuchsia system images',
     'pattern': '.',
-    'condition': 'run_fuchsia_emu',
+    'condition': 'download_fuchsia_deps and run_fuchsia_emu',
     'action': [
       'env',
       'DOWNLOAD_FUCHSIA_SDK={download_fuchsia_sdk}',

ci/builders/linux_fuchsia.json

@@ -5,9 +5,6 @@
                 "device_type=none",
                 "os=Linux"
             ],
-            "gclient_variables": {
-                "download_android_deps": false
-            },
             "gn": [
                 "--fuchsia",
                 "--fuchsia-cpu",
@@ -31,7 +28,6 @@
                 "os=Linux"
             ],
             "gclient_variables": {
-                "download_android_deps": false,
                 "run_fuchsia_emu": true
             },
             "gn": [
@@ -68,9 +64,6 @@
                 "device_type=none",
                 "os=Linux"
             ],
-            "gclient_variables": {
-                "download_android_deps": false
-            },
             "gn": [
                 "--fuchsia",
                 "--fuchsia-cpu",
@@ -94,7 +87,6 @@
                 "os=Linux"
             ],
             "gclient_variables": {
-                "download_android_deps": false,
                 "run_fuchsia_emu": true
             },
             "gn": [
@@ -131,9 +123,6 @@
                 "device_type=none",
                 "os=Linux"
             ],
-            "gclient_variables": {
-                "download_android_deps": false
-            },
             "gn": [
                 "--fuchsia",
                 "--fuchsia-cpu",
@@ -173,7 +162,6 @@
                 "os=Linux"
             ],
             "gclient_variables": {
-                "download_android_deps": false,
                 "run_fuchsia_emu": true
             },
             "gn": [
@@ -210,9 +198,6 @@
                 "device_type=none",
                 "os=Linux"
             ],
-            "gclient_variables": {
-                "download_android_deps": false
-            },
             "gn": [
                 "--fuchsia",
                 "--fuchsia-cpu",
@@ -236,7 +221,6 @@
                 "os=Linux"
             ],
             "gclient_variables": {
-                "download_android_deps": false,
                 "run_fuchsia_emu": true
             },
             "gn": [
@@ -273,10 +257,6 @@
                 "device_type=none",
                 "os=Linux"
             ],
-            "gclient_variables": {
-                "download_android_deps": false,
-                "run_fuchsia_emu": true
-            },
             "gn": [
                 "--fuchsia",
                 "--fuchsia-cpu",
@@ -300,7 +280,6 @@
                 "os=Linux"
             ],
             "gclient_variables": {
-                "download_android_deps": false,
                 "run_fuchsia_emu": true
             },
             "gn": [
@@ -337,10 +316,6 @@
                 "device_type=none",
                 "os=Linux"
             ],
-            "gclient_variables": {
-                "download_android_deps": false,
-                "run_fuchsia_emu": true
-            },
             "gn": [
                 "--fuchsia",
                 "--fuchsia-cpu",
@@ -380,7 +355,6 @@
                 "os=Linux"
             ],
             "gclient_variables": {
-                "download_android_deps": false,
                 "run_fuchsia_emu": true
             },
             "gn": [