Update macOS GHA runner to aarch64

[penelopeysm]

cf. TuringLang/Turing.jl#2395

Also just reworks the CI matrix to be clearer

[coveralls]

Pull Request Test Coverage Report for Build 12053991282

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 25 unchanged lines in 4 files lost coverage.
• Overall coverage remained the same at 84.354%

---

Files with Coverage Reduction	New Missed Lines	%
src/model.jl	1	93.68%
src/varinfo.jl	6	86.3%
src/simple_varinfo.jl	6	86.6%
src/threadsafe.jl	12	57.76%

Totals
Change from base Build 12053405262:	0.0%
Covered Lines:	3553
Relevant Lines:	4212

---

💛 - Coveralls

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.35%. Comparing base (5bc980a) to head (5b31ce3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #724   +/-   ##
=======================================
  Coverage   84.35%   84.35%           
=======================================
  Files          35       35           
  Lines        4212     4212           
=======================================
  Hits         3553     3553           
  Misses        659      659           

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[penelopeysm]

CI failure is unrelated and I've had no luck trying to reproduce it.

Going to merge because this is a boring change which was already discussed on Turing.jl repo