CI: Restore MacOS compiler variables

[marshallward]

GitHub appears to have restored their compiler alias conventions on
their MacOS nodes. This patch restores those variables.

[codecov]

Codecov Report

Merging #191 (fe79250) into dev/gfdl (4d93e3a) will not change coverage.
The diff coverage is n/a.

❗ Current head fe79250 differs from pull request most recent head 968c3dc. Consider uploading reports for the commit 968c3dc to get more accurate results

@@            Coverage Diff            @@
##           dev/gfdl     #191   +/-   ##
=========================================
  Coverage     37.22%   37.22%           
=========================================
  Files           261      261           
  Lines         72319    72319           
  Branches      13533    13533           
=========================================
  Hits          26921    26921           
  Misses        40407    40407           
  Partials       4991     4991           

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