Try ARM64 runners for mutation tests

[martincostello]

Try out running the mutation tests on the new Ubuntu ARM64 runner as an immediate follow-on to #2520 to see if it gets faster still.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.22%. Comparing base (4083d90) to head (98d0da4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2523   +/-   ##
=======================================
  Coverage   94.22%   94.22%           
=======================================
  Files         311      311           
  Lines        7413     7413           
  Branches     1038     1038           
=======================================
  Hits         6985     6985           
  Misses        335      335           
  Partials       93       93           

Flag	Coverage Δ
linux	94.22% <ø> (ø)
macos	94.22% <ø> (ø)
windows	94.21% <ø> (ø)

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[martincostello]

At the time of writing this doesn't appear to give us any tangible improvement (it's actually slower):

Commit	Core	Extensions	Legacy	RateLimiting	Testing
4083d90 (main)	25m 48s	2m 59s	6m 34s	2m 38s	2m 52s
98d0da4 (this PR)	29m 51s	3m 37s	9m 54s	3m 21s	3m 37s

We can try again when the ARM64 runner comes out of preview in case any wrinkles get ironed out.

There seems to be some degree of variability anyway as #2520 was 19m 19s for Polly.Core, but after merged it was slower at 25m 48s for the same code.