copr: builders: don't re-download all metadata when spawning

If we set metadata_expire=1h, it is effectively the same as before
(metadata will be downloaded, when updated).  But using zchunk in
F30+ we can (at least right after the image is freshly created)
safe some download bandwidth.  Btw., aarch64 used to spent like
120s+ on re-downloading everything.

roles/copr/backend/files/provision/provision_builder_tasks.yml

@@ -27,6 +27,13 @@
 
 - name: clean dnf cache
   shell: dnf clean all
+  when:
+    - prepare_base_image is defined
+
+- name: set lower metadata expire time to enforce download
+  ini_file: dest=/etc/dnf/dnf.conf section=main option=metadata_expire value=1h
+  when:
+    - prepare_base_image is defined
 
 - name: install copr-builder
   dnf: state=latest pkg=copr-builder