Fixes <http://bugs.gnu.org/26975>.
Reported by Mark H Weaver <mhw@netris.org>.
* guix/scripts/publish.scm (bake-narinfo+nar): Pass #f as the 2nd
argument to 'stat' and properly handle #f.
* tests/publish.scm (wait-for-file): New procedure.
("with cache"): Remove 'wait-for-file' procedure.
("with cache, uncompressed"): New test.