Change GHA macOS architecture to aarch64

[penelopeysm]

A few months ago (I think) GitHub updated their macos-latest runners to use Apple Silicon.

Since then, we've apparently been testing x64 Julia on an ARM runner which basically runs Julia via Rosetta emulation.

This PR fixes it.

cf. JuliaLang/www.julialang.org#2196 and JuliaLang/julia#55878

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.41%. Comparing base (5b24ceb) to head (5a819ae).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2395   +/-   ##
=======================================
  Coverage   86.41%   86.41%           
=======================================
  Files          22       22           
  Lines        1575     1575           
=======================================
  Hits         1361     1361           
  Misses        214      214           

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[mhauru]

Nice catch! Does this need to be changed in other repos too?

[penelopeysm]

Indeed it does

[coveralls]

Pull Request Test Coverage Report for Build 12051601116

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.585%

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Totals
Change from base Build 11725449698:	0.0%
Covered Lines:	1233
Relevant Lines:	1569

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💛 - Coveralls