fedora-cosmic: add OBS network:idm repo for Kanidm packages

Kanidm is not in the Fedora repos; it ships from the openSUSE OBS
network:idm project. Add per-distro repo files (Fedora_44 / Fedora_Rawhide)
and teach build.sh to overlay repos/<distro>/*.repo.

images/fedora-cosmic/manifest.yaml

@@ -21,6 +21,9 @@ ref: fedora-cosmic/${releasever_ref}/${basearch}/cosmic
 repos:
   # Local repo generated by build.sh from the token2-fido-bridge GitHub release.
   - token2-fido-bridge
+  # Kanidm packages (OBS network:idm); distro-specific repo file selected by
+  # build.sh from repos/<distro>/.
+  - network_idm
 
 include:
   - cosmic-atomic.yaml