test(.gcb/scripts): replace sidekick with librarian

[julieqiu]

This is a test.

For googleapis/librarian#3219

[julieqiu]

/gcbrun

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.74%. Comparing base (cb1255b) to head (9f2dbf8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4135   +/-   ##
=======================================
  Coverage   94.74%   94.74%           
=======================================
  Files         184      184           
  Lines        6926     6926           
=======================================
  Hits         6562     6562           
  Misses        364      364           

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