Add multi-site plugin validation back

The multi-site plugin validation was accidentally
removed with Id1f5f80417b75,

Change-Id: I2423718f15f8643429667c82bc81dd676a9ea4cd
diff --git a/jenkins/gerrit-bazel-plugin-multi-site.yaml b/jenkins/gerrit-bazel-plugin-multi-site.yaml
index 4b4593c..1111e60 100644
--- a/jenkins/gerrit-bazel-plugin-multi-site.yaml
+++ b/jenkins/gerrit-bazel-plugin-multi-site.yaml
@@ -8,3 +8,4 @@
             - stable-3.3
             - stable-3.2
             - stable-2.16
+      - 'plugin-{name}-verifier'