Scheduled functions use 'serviceAccount' as the invoker if provided

[inlined]

Fix for #6814

Previously we always used the default compute service account for the calls from Cloud Scheduler to Cloud Functions. This is unnecessary, as the user will now require actAs permissions on possibly two service accounts, and can cause errors if this default compute service account has been deleted.

Now, when serviceAccount is specified on the function, it is used both as the service account the container runs as, the valid invoker for the function, and the service account Cloud Scheduler uses to invoke the function. This slightly shifts an edge case regarding which service account the Cloud Scheduler service agent must have actAs permissions over, but that IAM concern has always existed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 54.27%. Comparing base (ede7d0b) to head (9f083f3).
Report is 492 commits behind head on master.

Files with missing lines	Patch %	Lines
src/deploy/functions/release/fabricator.ts	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6858      +/-   ##
==========================================
+ Coverage   54.24%   54.27%   +0.02%     
==========================================
  Files         352      352              
  Lines       24531    24534       +3     
  Branches     5079     5082       +3     
==========================================
+ Hits        13307    13315       +8     
+ Misses      10009    10006       -3     
+ Partials     1215     1213       -2     

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