rpmbuild: fix the check if a priority was set

We are setting `priority=None` in the `dnf.conf` and getting:

    Invalid configuration value: priority= in
    /var/lib/mock/fedora-38-x86_64-bootstrap-1691688959.249909/root/etc/dnf/dnf.conf;
    invalid value

rpmbuild/mock.cfg.j2

@@ -52,7 +52,7 @@ metadata_expire=0
 cost=1
 best=1
 
-{%- if "priority" in repo and priority is not none %}
+{%- if repo.get("priority") is not none %}
 priority={{ repo["priority"] }}
 {%- endif %}