Alternative to #33562, which was adding a native FreeBSD job; however that had issues with permissions/caching, as well as potential determinism issues. This adds a FreeBSD cross job using Linux and Clang.
This is using FreeBSD 14,3, which avoids the issue in #34093.
Would close #33438. The same changes here could also be used to produce FreeBSD binaries out of Guix.