mrhlpr.mr: Also include systemd repo when upgrading

Otherwise apk won't fetch packages from the systemd extra repo when
using `mrtest upgrade`.

mrhlpr/mr.py

@@ -180,8 +180,11 @@ def get_artifacts_repo_urls(
     else:
         pmos_release = _target_branch_to_pmos_release(api["target_branch"])
 
+        # FIXME: This is just hard-coding extra repos. While we aren't likely to add
+        # more ones in the future, it'd be better to not do this.
         return [
             f"{host_arch_job_web_url}/artifacts/raw/packages/{pmos_release}",
+            f"{host_arch_job_web_url}/artifacts/raw/packages/systemd-{pmos_release}",
         ]