Fix GCS initialization

[msuozzo]

GCS usage currently fails as the initializtion path happens outside of a
Tokio runtime context.

[msuozzo]

I have a separate implementation which actually colors the callers as async but it's necessarily a lot bigger a change so I figured I'd start here.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.38%. Comparing base (2b65ac8) to head (0f2d138).

Files with missing lines	Patch %	Lines
src/cache/gcs.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2637      +/-   ##
==========================================
+ Coverage   73.37%   73.38%   +0.01%     
==========================================
  Files          68       68              
  Lines       37337    37338       +1     
==========================================
+ Hits        27395    27400       +5     
+ Misses       9942     9938       -4     

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