Support build from commit sha instead of release version

[Matthew-Benson]

Users may want to pull Fish source with a specific commit ref instead of a release tag. Since there's no release archive, we would have to do a git repo fetch, but I believe there's a built in bazel lib function to pull a git repo.

To support this, users may have to provide some additional configuration like whether they want to use the c++ toolchain or Rust toolchain since we may not be able to infer from a release number.