Before that, /nar requests could succeed if the requested store item
exists but is invalid (although such requests were unlikely because the
corresponding narinfo request would have failed.)
* guix/scripts/publish.scm (render-nar): Add 'store' parameter. Use
'valid-path?' instead of 'file-exists?'.
(make-request-handler): Adjust 'render-nar' call accordingly.
* tests/publish.scm ("/nar/invalid"): New test.
