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

[zijiehe-google-com]

Reverts #51500 and relands #51439 with the default download_fuchsia_deps unchanged and enabled via host_os == "linux".

Something not mentioned in the previous change description, since the engine_v2 recipe now supports gclient_variables forwarding, I moved the duplicated download_android_deps: False into the .ci.yaml, i.e. the root builder from the drone ci/builders/linux_fuchsia.json file.

Original change description:

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

[zijiehe-google-com]

A little bit struggling to demonstrate the importance of this change; keep it as a draft for now.

The only benefit of this change is to avoid downloading images in two of eight drone builders which was unfortunately caused by a typo in the original change.

[jtmcdole]

(triage): reached out to author for more feedback

[zijiehe-google-com]

Thank you for reaching out. As mentioned in #51508 (comment), this change isn't important enough to merge.