packit test: switch to 'legacy-ci' branch

The main branch switched to the mkosi runner, so we need to use
the older integration code for stable branches

Author: bluca

.packit.yml

@@ -55,7 +55,7 @@ jobs:
 - job: tests
   trigger: pull_request
   fmf_url: https://src.fedoraproject.org/tests/systemd
-  fmf_ref: main
+  fmf_ref: legacy-ci
   tmt_plan: ci
   targets:
   - fedora-40-x86_64