Add support for FreeBSD runners

[galargh]

This would be useful for testing in quic-go

Current state of FreeBSD support:

• FreeBSD support actions/runner#385 <- looks like it's quite unlikely GitHub is going to release a runner binary for BSD systems
• https://github.com/ChristopherHX/github-act-runner <- there are alternative runner implementations that claim to work on BSD (possibly with limited functionality)
• https://github.com/marketplace/actions/cross-platform-action <- there is an action that can run BSD VMs on hosted GitHub Actions runners (at a time penalty of course)

[marten-seemann]

Just as an example, this probably would have allowed us to catch quic-go/quic-go#4106.

[galargh]

ACK 🙇 I would like to have the Windows runners ready by the end of the month. I really do want to check https://github.com/ChristopherHX/github-act-runner out too but I cannot commit to that now, unfortunately 😢

[marten-seemann]

Not being able to FreeBSD is having quite drastic consequences. Multiple users have had to disable the QUIC transport: Celestia and Adguard, for example. There might be more that I'm not aware of.

Is there any other way you see to test on FreeBSD?

[galargh]

I can't promise anything but I'll try to give https://github.com/ChristopherHX/github-act-runner a go this week (if only nucleation permits).

As a quick win, I think we could try https://github.com/marketplace/actions/cross-platform-action. Is it https://github.com/quic-go/quic-go/blob/master/.github/workflows/integration.yml that would be most beneficial if we could run it on FreeBSD too?