Test on NetBSD and OpenBSD

[notgull]

Given that there are some LibXCB-related exceptions on NetBSD and OpenBSD, we should be able to test them.

We can use setup-cross-toolchain-action to test NetBSD, but it doesn't support OpenBSD yet.

[psychon]

I doubt that a cross toolchain would actually catch this, since this would need the distro's libxcb.so available and this is actually about the dl-libxcb feature, which only fails at runtime. So this could only be tested on the actual OSes.

Also, I'm not sure whether this is only broken on NetBSD and OpenBSD. I would rather guess that those are the only "weird OSes" that people use that actually ship with libxcb.so (which Windows does not and Mac OS likely does not).

[notgull]

The action I linked above creates a VM and runs the cargo actions via qemu-user. So it should be enough to really effectively test libxcb.