feat: Add SCCACHE_GCS_CREDENTIALS_URL feature back for gcs

[Xuanwo]

Signed-off-by: Xuanwo github@xuanwo.io

This PR will add SCCACHE_GCS_CREDENTIALS_URL feature back so that Mozilla can use sccache in CI without changing existing config.

[codecov-commenter]

Codecov Report

Base: 30.06% // Head: 29.93% // Decreases project coverage by -0.12% ⚠️

Coverage data is based on head (7d4cade) compared to base (ddb057a).
Patch coverage: 8.10% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1494      +/-   ##
==========================================
- Coverage   30.06%   29.93%   -0.13%     
==========================================
  Files          47       47              
  Lines       16642    16676      +34     
  Branches     7873     7900      +27     
==========================================
- Hits         5003     4992      -11     
- Misses       6328     6366      +38     
- Partials     5311     5318       +7     

Impacted Files	Coverage Δ
src/cache/gcs.rs	10.86% <0.00%> (-18.55%)	⬇️
src/cache/cache.rs	36.18% <50.00%> (+0.89%)	⬆️
src/config.rs	35.97% <50.00%> (+0.33%)	⬆️
src/compiler/gcc.rs	54.85% <0.00%> (-0.73%)	⬇️
src/server.rs	31.67% <0.00%> (-0.53%)	⬇️
src/lib.rs	8.76% <0.00%> (-0.35%)	⬇️
src/util.rs	33.76% <0.00%> (-0.33%)	⬇️
src/compiler/msvc.rs	43.42% <0.00%> (-0.27%)	⬇️
src/compiler/compiler.rs	35.84% <0.00%> (-0.15%)	⬇️
... and 8 more

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[sylvestre]

Looks like it fixed Firefox CI:
https://firefoxci.taskcluster-artifacts.net/THQ6jVgKSf-gL9XiYaxKSQ/0/public/build/sccache.log