packit.yaml: fix upstream tag template

Currently the version in fedora is `v55`, which is wrong, it should just
be `55`.

packit.yaml

@@ -4,6 +4,7 @@ upstream_package_name: cockpit-image-builder
 downstream_package_name: cockpit-image-builder
 # use the nicely formatted release description from our upstream release, instead of git shortlog
 copy_upstream_release_description: true
+upstream_tag_template: v{version}
 
 actions:
   create-archive: