mkosi: disable repository key check on Fedora

Every six months when a new Fedora release is branched, key verification breaks.
Given these images are thrown away and are only for CI purposes, disable key
verification.

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>

Author: bluca

mkosi/mkosi.conf.d/fedora/mkosi.conf

@@ -1,6 +1,9 @@
 [Match]
 Distribution=fedora
 
+[Distribution]
+RepositoryKeyCheck=no
+
 [Content]
 Packages=
         sbsigntools