Add FreeBSD 12.2 vagrant image to CI
https://github.com/haskell/haskell-language-server/pull/1729#issuecomment-921713194 suggests we should build on 12.2 for wider compatibility for now?
In case 12.2 runs smoothly on vagrant, we could re-use the runner for both FreeBSD versions.
Whether GHC bindists itself should be built on 12.2, 13 or both, I'm not sure. Opinions?