ci: run unit and integration tests on freebsd

[galargh]

Follow up on ipdxco/custom-github-runners#24

This PR adds FreeBSD builds support (unit + integration) using https://github.com/cross-platform-actions/action (the action runs FreeBSD in a VM; using macos provides faster builds).

Providing native FreeBSD self-hosted runners would require using unofficial runner binary (https://github.com/ChristopherHX/github-act-runner). Looking through the description of the repo, that can sometimes result in irrecoverable breakage of GitHub Actions in a repo where such runner is used which we'd definitely want to avoid. That's why I decided using VM action is safer. We could still look into running VMs on self-hosted machines to speed up the builds though.

Issues I noticed when using the action:

• sometimes the builds get stuck in go download phase
• sometimes hd attachment that the action does fails on hosted runners

Testing:

• https://github.com/galargh/quic-go/actions/runs/6628996738
• https://github.com/galargh/quic-go/actions/runs/6628996737

Unit test failures:

• [FAIL] HTTP 0.9 integration tests [It] allows setting of headers
• [FAIL] HTTP 0.9 integration tests [It] performs request

Integration test failures:

• HTTP tests [It] * (self tests, using QUIC v1)
• HTTP tests [It] *(self tests, using QUIC v2)