Avoid passing PEX_MAX_INSTALL_JOBS incorrectly to PEX processes (#20795)

This fixes/removes the use of `PEX_MAX_INSTALL_JOBS` when running a PEX.
This was added in #20670 in an attempt to sync with
`--no-pre-install-wheels` and do more work in parallel when booting
"internal" PEXes... but it was implemented incorrectly. This would need
to be set to `PEX_MAX_INSTALL_JOBS={pants_concurrency}` or similar, and:

- that substitution isn't currently supported (only argv substitutions)
- it's somewhat unclear if we even want to do that at all, as it'll
result in more cache misses

Noticed in
#20670 (comment)

src/python/pants/backend/python/goals/publish_test.py

@@ -140,7 +140,6 @@ def test_twine_upload(rule_runner, packages) -> None:
                 {
                     "TWINE_USERNAME": "whoareyou",
                     "TWINE_PASSWORD": "secret",
-                    "PEX_MAX_INSTALL_JOBS": "0",
                 }
             ),
         ),
@@ -162,7 +161,7 @@ def test_twine_upload(rule_runner, packages) -> None:
                 "my-package-0.1.0.tar.gz",
                 "my_package-0.1.0-py3-none-any.whl",
             ),
-            env=FrozenDict({"TWINE_USERNAME": "whoami", "PEX_MAX_INSTALL_JOBS": "0"}),
+            env=FrozenDict({"TWINE_USERNAME": "whoami"}),
         ),
     )
 

src/python/pants/backend/python/util_rules/pex.py

@@ -1163,9 +1163,6 @@ async def setup_pex_process(request: PexProcess, pex_environment: PexEnvironment
     complete_pex_env = pex_environment.in_sandbox(working_directory=request.working_directory)
     argv = complete_pex_env.create_argv(pex.name, *request.argv)
     env = {
-        # Set this in case this PEX was built with --no-pre-install-wheels, and thus parallelising
-        # the install on cold boot is handy.
-        "PEX_MAX_INSTALL_JOBS": str(request.concurrency_available),
         **complete_pex_env.environment_dict(python=pex.python),
         **request.extra_env,
     }
@@ -1261,12 +1258,6 @@ async def setup_venv_pex_process(
         else venv_pex.pex.argv0
     )
     argv = (pex_bin, *request.argv)
-    env = {
-        # Set this in case this PEX was built with --no-pre-install-wheels, and thus parallelising
-        # the install on cold boot is handy.
-        "PEX_MAX_INSTALL_JOBS": str(request.concurrency_available),
-        **request.extra_env,
-    }
     input_digest = (
         await Get(Digest, MergeDigests((venv_pex.digest, request.input_digest)))
         if request.input_digest
@@ -1285,7 +1276,7 @@ async def setup_venv_pex_process(
         level=request.level,
         input_digest=input_digest,
         working_directory=request.working_directory,
-        env=env,
+        env=request.extra_env,
         output_files=request.output_files,
         output_directories=request.output_directories,
         append_only_caches=append_only_caches,