CI: Test both incremental={true,false} on "slow CI"; also, stop considering Apple Silicon macOS as "slow CI"

[DilumAluthge]

In my recent CI experiments, I've discovered some bugs that only occur in one branch of incremental={true,false}. So I think we should try to exercise both of those branches in CI, even if we are in "slow CI".

[DilumAluthge]

I did a bit of Git archeology. As far as I can tell, the is_slow_ci flag was introduced before we had Apple Silicon. I wonder if we maybe had Linux aarch64 CI back then, and that's what the is_slow_ci flag was meant to cover.

AFAICT, the Apple Silicon GitHub Actions CI GitHub-hosted runners on this repo seem to run the CI jobs pretty quickly.

So I don't think Apple Silicon should count as "slow CI".

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.56%. Comparing base (f5eba5f) to head (14f260e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #983   +/-   ##
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  Coverage   84.56%   84.56%           
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  Files           3        3           
  Lines         823      823           
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  Hits          696      696           
  Misses        127      127           

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