FreeBSD: Snap images are not supported due to boot stability.

[igsilya]

All our jobs started failing with:

Failed to start an instance: INVALID_ARGUMENT: Snap images are not supported due to boot stability.

We use image_family: freebsd-14-3-snap as we had issues with pkg update on release images in the past. we can try to go back to release images, i.e. image_family: freebsd-14-3 and hope that it works fine (works for now), but the restriction should probably be documented, as least. Though I'm not sure if the failure is coming from Cirrus CI or from the GCE...

[asomers]

Please consider restoring this functionality. It's especially important for FreeBSD's main branch (currently FreeBSD 16), which doesn't yet have a stable release.