post-merge plugins use default canvas_lms_refspec

Post-merge plugin builds should run against canvas master.

flag = none

Test Plan:
-run plugin build with custom canvas-lms-refspec, view logs
  to see that it was honored.
-run change-merged plugin build, verify the default master
  canvas-lms-refspec was used rather than the custom refspec.

Change-Id: Ia96628543e2e0d8a9f16225f787e59e0be481094
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/249597
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Aaron Ogata <aogata@instructure.com>
Product-Review: James Butters <jbutters@instructure.com>
QA-Review: James Butters <jbutters@instructure.com>

Jenkinsfile

@@ -228,6 +228,9 @@ def getExternalTag() {
 def getDependenciesImage() {
   return env.GERRIT_EVENT_TYPE == 'change-merged' ? configuration.dependenciesImageDefault() : configuration.dependenciesImage()
 }
+def getCanvasLmsRefspec() {
+  return env.GERRIT_EVENT_TYPE == 'change-merged' ? configuration.canvasLmsRefspecDefault() : configuration.canvasLmsRefspec()
+}
 // =========
 
 pipeline {
@@ -276,7 +279,7 @@ pipeline {
     POSTGRES_IMAGE_TAG=imageTag.postgres()
     // This is primarily for the plugin build
     // for testing canvas-lms changes against plugin repo changes
-    CANVAS_LMS_REFSPEC=configuration.canvasLmsRefspec()
+    CANVAS_LMS_REFSPEC = getCanvasLmsRefspec()
     DOCKER_WORKDIR = getDockerWorkDir()
     LOCAL_WORKDIR = getLocalWorkDir()
   }