Cirrus CI for FreeBSD builds

[hdholm]

I don't know if there is any interest in this. It seems that GitHub has no interest in supporting FreeBSD runners, but Cirrus CI does. It requires a (free for open source) Cirrus CI account and installing the CirrusCI application for the repository. This PR is for the config file to run FreeBSD tests (and for github actions to ignore changes to the config since it won't affect the GitHub actions.)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (0210575) 58.65% compared to head (b15dc86) 58.65%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #146   +/-   ##
=======================================
  Coverage   58.65%   58.65%           
=======================================
  Files           4        4           
  Lines         578      578           
  Branches      147      147           
=======================================
  Hits          339      339           
  Misses        138      138           
  Partials      101      101           

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