tests: Track Cuirass' git.

* guix.scm: New file; specify Guix package.
* guix/ci.scm: New file; expose to Guix.
* build-aux/pre-inst-env.in: Add it to GUIX_PACKAGE_PATH.
* bin/evaluate.in (main): Lookup proc using name specified by #:proc.
* tests/guix-track-git.scm: New file.
* tests/hello-git.scm: Test it.

Signed-off-by: Mathieu Lirzin <mthl@gnu.org>