.github/workflows: activate freebsd build on PRs

[gballet]

No description provided.

[fjl]

Build failed in hidapi

[gballet]

This isn't the same error as the one that was reported in that issue though. actually it is, I thought it was a library thing but it's an include thing.

So I dug in a little, and the thing is that the ports build uses the ports version of the library. This is why the fix proposed by Enrique works for building ports. As such, the fix proposed in #34632 is wrong, as ports and system might have a different version of the headers.

[s1na]

I'd recommend a way to cancel a job if there were subsequent commits to the same PR. Otherwise it will just enqueue multiple (most of which is obsolete) jobs.

  concurrency:
    group: freebsd-${{ github.workflow }}-${{ github.ref }}
    cancel-in-progress: true

[fjl]

Please ensure we don't use external GH actions repository for launching the VM, i.e. fork it to your own account or the ethereum org, and make sure all JS dependencies are pinned.

We should only activate this when we are sure the external action is safe.