Parent legacy-gerrit-buck to legacy-base

This currently fails with

  Unable to freeze job graph: Pre-review pipeline check does not allow
  post-review job legacy-gerrit-buck

because the artifacts parent is a protected job, so can't be in the
check pipeline.  I think what's happening here is that this job just
tests the build of the packages, and the -publish variant actually
wants to publish it.

Change-Id: I2111a29e28d091a365e65936ec34a909d8cce980

zuul.d/zuul-legacy-jobs.yaml

@@ -307,7 +307,7 @@
 
 - job:
     name: legacy-gerrit-buck
-    parent: legacy-publish-openstack-artifacts
+    parent: legacy-base
     run: playbooks/legacy/gerrit-buck/run.yaml
     post-run: playbooks/legacy/gerrit-buck/post.yaml
     timeout: 2400