Install ruby commands on windows

[scpeters]

🦟 Bug fix

Part of gazebosim/gz-tools#71.

Summary

Make sure to add_subdirectory(cmd) on Windows. This requires generating the .rb and .yaml files to folders unique to $<CONFIG>.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[codecov]

Codecov Report

Merging #63 (06ccbae) into ign-plugin1 (e4015f3) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-plugin1      #63   +/-   ##
============================================
  Coverage        99.82%   99.82%           
============================================
  Files               15       15           
  Lines              584      584           
============================================
  Hits               583      583           
  Misses               1        1           

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