lint: formatting: Gracefully handle relative file names.

Fixes <https://bugs.gnu.org/42543>.
Reported by Jack Hill <jackhill@jackhill.us>.

* guix/lint.scm (check-formatting): Always return a list (previously we
would return #f when 'search-path' returns #f).  Check whether
LOCATION's file is a relative file name.  Return a warning if not.
* tests/guix-lint.sh: Add test.