As long as there's some envvars with the SHA, branch name, remote, etc, all that should be handleable by a wrapper (or git itself) being able to fall back on those in instances where it's invoked in a tarball of a repo rather than a real repo.

EDIT: Or alternatively (and probably better), the forges could include a dummy .git directory in the tarball that declares it an "archive"-type clone (vs shallow or full), and the git client would read that and offer the same unshallow/fetch/etc options that are available to a regular shallow clone.