Fixes <https://bugs.gnu.org/28664>.
Reported by Maxim Cournoyer <maxim.cournoyer@gmail.com>.
* guix/scripts/publish.scm (bake-narinfo+nar): When COMPRESSION is
'none, write NAR instead of doing nothing.
(make-request-handler): Use 'render-nar/cached' for /nar URLs with no
compression.
* tests/publish.scm ("with cache", "with cache, uncompressed"): Adjust
expected result accordingly.