Add freebsd testing to CI

[UebelAndre]

We need a way to test on freebsd in CI but that doesn't seem to be a supported platform. Having this platform in CI means we could more confidently merge #387 and other such changes.

[UebelAndre]

bazelbuild/continuous-integration#258 seems relevant. Also, a list of supported platforms can be found in @continuous-integration//buildkite/bazelci.py

[github-actions]

This issue has been automatically marked as stale because it has not had any activity for 180 days. It will be closed if no further activity occurs in 30 days. Collaborators can add an assignee to keep this open indefinitely. Thanks for your contributions to rules_foreign_cc!

[github-actions]

This issue was automatically closed because it went 30 days without a reply since it was labeled "Can Close?"

[yesudeep]

no, bot! FreeBSD rocks. that low latency, highly responsive OS needs a bit of love. re-opening!

[github-actions]

This issue has been automatically marked as stale because it has not had any activity for 180 days. It will be closed if no further activity occurs in 30 days. Collaborators can add an assignee to keep this open indefinitely. Thanks for your contributions to rules_foreign_cc!

[github-actions]

This issue was automatically closed because it went 30 days without a reply since it was labeled "Can Close?"