Fetch GerritHub.io credentials for the scripting plugins build

Change-Id: I29cdcc1c4e8df0bfaf625e5b7a8d79d2f7ad2ce1

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

diff --git a/jenkins/gerrit-bazel-plugin-scripting-template.yaml b/jenkins/gerrit-bazel-plugin-scripting-template.yaml
index 6d60cf9..62051b9 100644
--- a/jenkins/gerrit-bazel-plugin-scripting-template.yaml
+++ b/jenkins/gerrit-bazel-plugin-scripting-template.yaml
@@ -12,6 +12,10 @@
               credential-id: gerrit.googlesource.com
               username: GS_GIT_USER
               password: GS_GIT_PASS
+          - username-password-separated:
+              credential-id: eclipse.gerrithub.io
+              username: GH_GIT_USER
+              password: GH_GIT_PASS
     properties:
       - build-discarder:
           days-to-keep: 20