ci: Fix version number of packit generated srpms

Our tags have a `v` prefix. Tell packit about it, so that it generates
correct `Version:` numbers in the spec.

packit.yaml

@@ -1,6 +1,7 @@
 # See https://packit.dev/docs/configuration/
 
 specfile_path: tmp/rpm/selinux-policy.spec
+upstream_tag_template: "v{version}"
 
 actions:
   post-upstream-clone: