Package workspace relocation

[Stephen Scheck]

Is there a way to relocate the package build workspace for a Git clone
source, e.g.:

    (package
      (name "my-package")
      (version "0.0.1")
      (source (origin
                      (method git-fetch)
                      (uri (git-reference
                         (url "https://github.com/my-package";)))
                         (file-name (git-file-name name version))

This results in a path like `<guix-build-temp>/my-package-0.0.1` but
because of idiosyncrasies in the project's build system, I need for it to
be relocated to something like `<guix-build-temp>/foo/bar/my-package-0.0.1`.

How can I do this?

Thanks.