Explicitly install dnf-plugins-core on Rocky 9

It is needed to get "dnf config-manager". It was implicitly installed
previously, but Red Hat changed things.

.github/containers/rocky9/Dockerfile

@@ -1,7 +1,7 @@
 FROM rockylinux:9
 
 RUN dnf -y group install 'Development Tools'
-RUN dnf -y install sudo
+RUN dnf -y install dnf-plugins-core sudo
 RUN dnf config-manager --set-enabled crb
 
 RUN useradd -s /bin/bash -m rpm