provide a CI pipeline for FreeBSD

[mmitch]

We got bitten by #105 on FreeBSD because we don't have regular build checks. The FreeBSD specific error seems to have been introduced by commit fad6a3e3b951de1f7def379a2628596916c3dad9 16 months ago.

It seems to be possible to run a CI pipeline with FreeBSD so we should check that out:

• https://github.com/marketplace/actions/freebsd-vm
• https://github.com/cross-platform-actions/action?tab=readme-ov-file#minimal-example