ci: enable CI and GCB triggers for grpc_rust

[joshuatants]

For #5991

[gemini-code-assist]

Code Review

This pull request updates the Google Cloud Build trigger configurations in .gcb/builds/triggers/main.tf to target both the main and grpc_rust branches for pull-request and post-merge events. I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.02%. Comparing base (a69f6d5) to head (4ddf286).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6055      +/-   ##
==========================================
- Coverage   97.03%   97.02%   -0.01%     
==========================================
  Files         252      252              
  Lines       63526    63526              
==========================================
- Hits        61640    61636       -4     
- Misses       1886     1890       +4     

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