Build a release asset for FreeBSD

[ararslan]

This uses the vmactions/freebsd-vm GitHub Action to build an asset for FreeBSD alongside the existing assets. See #438 for context.

The build steps as written produce a working binary on my FreeBSD 14.0 x86_64 system. Note however that the VM used for CI is FreeBSD 13.2; this is because binaries built on a FreeBSD system are forward- but not backward-compatible with other FreeBSD versions. That is, an executable built on 14.0 wouldn't necessarily work on 13.2, but one built on 13.2 will work on 14.0. The choice of 13.2 in particular is because it's the oldest currently supported release.

Python 3.11 is used for consistency with other platforms. Unlike other platforms, this build utilizes a virtual environment, simply because it's easier and less error prone.