CI: enhance FreeBSD staging tests (#2372)

CI: enhance FreeBSD staging tests (#2372)

Cover all FreeBSD versions supported by the FreeBSD Project.

Fully rely on packages, not ports, solving the "version mismatch"
problem that had led us to remove FreeBSD 13.5 in commit 99fca3a.

Enable ccache for faster build times.

Restrict testing to the maximus test layer to maximize
signal-to-execution-time ratio: The main purpose of FreeBSD tests is to
quickly identify Linux-isms and portability issues, not internal code
dependencies and misalignments.

Prepare to also support arm64 and riscv64, but leave them disabled for
now as they are respectively too slow and broken at this time.