gcs: default external_account credential format to text in memory

[akasakariko]

reqsign-google 3.1.0 cannot load external_account credentials when credential_source omits format, although AIP-4117 defaults it to text
For credentials referenced by SCCACHE_GCS_KEY_PATH or GOOGLE_APPLICATION_CREDENTIALS, sccache now inserts the default text format in memory before passing them to OpenDAL without rewriting the original file
Explicit formats remain untouched, empty environment paths are ignored, and an upstream fix is tracked in apache/reqsign#910

Fixes #2884

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.11321% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.75%. Comparing base (214cde1) to head (7a85e21).

Files with missing lines	Patch %	Lines
src/cache/gcs.rs	98.11%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2889      +/-   ##
==========================================
+ Coverage   77.66%   77.75%   +0.08%     
==========================================
  Files          72       72              
  Lines       41574    41732     +158     
==========================================
+ Hits        32290    32450     +160     
+ Misses       9284     9282       -2     

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[akasakariko]

Addressed, PTAL. Thanks!