Improve nightly CI

[mwestphal]

• fix macos arm64 CI CI: Use macOS M1 runner for arm CI #1240
• run nightly only on f3d-app/f3d CI: Disable nightly builds on forks #1236

[mwestphal]

@kidharb FYI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3f8eb1d) 96.59% compared to head (d05f18e) 96.57%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1282      +/-   ##
==========================================
- Coverage   96.59%   96.57%   -0.02%     
==========================================
  Files         138      138              
  Lines        8446     8494      +48     
==========================================
+ Hits         8158     8203      +45     
- Misses        288      291       +3     

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