Don't specify arch in ci matrix

[jishnub]

Currently, we specify the arch as x64, but the macos tests are run on arm64 platforms. It's perhaps best to leave this implicit so that the correct defaults are used.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.68%. Comparing base (e9d199c) to head (6ae42c6).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
- Coverage   89.93%   89.68%   -0.26%     
==========================================
  Files          25       25              
  Lines        3675     3673       -2     
==========================================
- Hits         3305     3294      -11     
- Misses        370      379       +9     

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