fix(cancel): follow through cancel on missing executor

[ecrupper]

If build is nil from CancelRunning, that doesn't mean we shouldn't go through with canceling. This will empower both auto cancel and other users to set a build status to canceled even if it's technically no longer accessible to the worker / in a weird state.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.01%. Comparing base (5b339d8) to head (9185927).

Files with missing lines	Patch %	Lines
api/build/cancel.go	0.00%	35 Missing ⚠️
api/build/auto_cancel.go	0.00%	9 Missing ⚠️
api/build/update.go	0.00%	2 Missing ⚠️

❌ Your project check has failed because the head coverage (58.01%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1441      +/-   ##
==========================================
- Coverage   58.06%   58.01%   -0.06%     
==========================================
  Files         663      663              
  Lines       25564    25588      +24     
==========================================
  Hits        14845    14845              
- Misses      10019    10043      +24     
  Partials      700      700              

Files with missing lines	Coverage Δ
api/build/update.go	0.00% <0.00%> (ø)
api/build/auto_cancel.go	32.85% <0.00%> (-4.24%)	⬇️
api/build/cancel.go	0.00% <0.00%> (ø)

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