CI: add FreeBSD build and test

[mwtoews]

Add CI coverage for building/testing on FreeBSD.

[mwtoews]

Confirming this draft implementation catches #1206:

...
  99% tests passed, 1 tests failed out of 510
  Total Test time (real) =   9.16 sec
  The following tests FAILED:
  	193 - unit-geom-Envelope (Failed)
  Errors while running CTest
  Error: The process '/usr/bin/ssh' failed with exit code 8

A bit more work to confirm that build .ccache dir can be reliably used...

[lbartoletti]

Many thanks :)