Allow GitHub plugin job to publish jar artifacts

The GitHub plugin CI template ignored the extra-artifacts argument, so
only the JSON metadata file was archived. As a result, github.jar and
github-oauth.jar were built but not exposed as artifacts by CI.

Change-Id: Ife7ac8b29b300a3b9f224d43a57ff3e7bb37830e

Author: syntonyze

jenkins/gerrit-bazel-plugin-gh-template.yaml

@@ -52,7 +52,8 @@
     publishers:
       - archive:
           artifacts: >
-            bazel-bin/plugins/{name}/{name}.jar*,bazel-bin/plugins/{name}/{name}.json
+            bazel-bin/plugins/{name}/{name}.jar*,bazel-bin/plugins/{name}/{name}.json,
+            {extra-artifacts}
           follow-symlinks: true
 
 - job-template: