Merge pull request #3615 from edx/jibsheet/add-github-pr-builder-plugin

We use to constrict our builds to PRs, similar to edx-platform

playbooks/roles/tools_jenkins/defaults/main.yml

@@ -36,6 +36,7 @@ jenkins_tools_plugins:
   - { name: "scm-api", version: "1.2" }
   - { name: "workflow-scm-step", version: "1.14.2" }
   - { name: "workflow-step-api", version: "1.14.2" }
+  - { name: "ghprb", version: "1.34.0" }
   - { name: "github-api", version: "1.77" }
   - { name: "git-client", version: "1.21.0"}
   - { name: "git", version: "2.5.3"}