Fixes <https://bugs.gnu.org/43736>.
Reported by Vitaliy Shatrov <guix.vits@disroot.org>.
* guix/gexp.scm (%local-file): Add #:literal? and #:location.
Emit a warning when LITERAL? is false and FILE is not absolute.
(local-file): In the non-literal case, pass #:location and #:literal?.
* po/guix/POTFILES.in: Add guix/gexp.scm.
* tests/guix-system.sh: Add test for the warning.