Add macos-12

[andrewdavidmackenzie]

Add macos-12 to the test matrix. Do coverage always

[codecov[bot]]

Codecov Report

Merging #76 (21f6339) into master (23c5257) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 21f6339 differs from pull request most recent head f396a4e. Consider uploading reports for the commit f396a4e to get more accurate results

@@           Coverage Diff           @@
##           master      #76   +/-   ##
=======================================
  Coverage   60.40%   60.40%           
=======================================
  Files           9        9           
  Lines         346      346           
=======================================
  Hits          209      209           
  Misses        137      137           

Impacted Files	Coverage Δ
src/libproc/pid_rusage.rs	29.41% <ø> (ø)
src/libproc/proc_pid.rs	78.15% <ø> (ø)

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