Run CI on FreeBSD again

[mobin-2008]

I wanted to work on configure fixes after the #550 but I realized that we lost our FreeBSD testing and I want to bring it back first.

Previously we used Cirrus CI for our FreeBSD testing, however Cirrus Labs joined OpenAI and shut down Cirrus CI in June 1st, 2026.

I had a little experiment with GitHub actions and now I have a working example of FreeBSD testing inside GitHub that

1. Uses KVM acceleration instead of QEMU TCG JIT compiler which yields near-native performance and I hope more accurate results
2. Only takes ~4 minutes from start to finish
3. Doesn't depend on any third-party actions except for the official actions/checkout and actions/upload-artifacts

Take a look at the example run of it: https://github.com/mobin-2008/dinit/actions/runs/28232281153/job/83638673157

[davmac314]

It would be good to have FreeBSD in CI, but I think we may as well make use of the existing freebsd-vm action (https://github.com/marketplace/actions/freebsd-vm) which will handle caching of the VM image and in general simplify the CI script a lot.

Edit: there's alsocross-platform-actions (https://github.com/cross-platform-actions/action) which can do other operating systems as well (and also caches images). It might be nice to have an OpenBSD or NetBSD build as well. (We can't have too many or we'll hit the cache limit).