Build macOS arm64 wheels on arm64 runner

[dstansby]

This should:

• Allow tests to be run for the arm64 wheels
• Speed up the wheel CI job a bit, as the {intel, arm64} macOS builds will now be done in parallel on different jobs.

[normanrz]

Should we run the build tests also on macos-14 as well? https://github.com/zarr-developers/numcodecs/actions/runs/9586809165/job/26435499989?pr=536

[dstansby]

👍 good idea

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (ecd8955) to head (9053439).
Report is 30 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #536   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          59       59           
  Lines        2318     2318           
=======================================
  Hits         2316     2316           
  Misses          2        2