Fix macOS workflow

[kevinbackhouse]

I noticed that the macOS build failed in this workflow: https://github.com/Exiv2/exiv2/actions/runs/1110704368
I have updated it to use the same steps as the on_PR_mac_matrix workflow.

[codecov]

Codecov Report

Merging #1862 (9c8d555) into 0.27-maintenance (bffbb14) will not change coverage.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           0.27-maintenance    #1862   +/-   ##
=================================================
  Coverage             46.21%   46.21%           
=================================================
  Files                   146      146           
  Lines                 22874    22874           
  Branches              11762    11762           
=================================================
  Hits                  10572    10572           
  Misses                 6696     6696           
  Partials               5606     5606           

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