add canvas-lms-refspec when running main-for-plugin build

flag = none
closes: DE-188

Test Plan:
    - Jenkins passes
    - setting canvas-lms-refspec in commit message
      properly propogates the env var
    - no setting the canvas-lms-refspec will use the
      default env var in jenkins config

Change-Id: I778cbbfc267fd8f2bf4d4a37937ee3417a6c846e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/245101
Reviewed-by: Aaron Ogata <aogata@instructure.com>
Product-Review: James Butters <jbutters@instructure.com>
Product-Review: Aaron Ogata <aogata@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: James Butters <jbutters@instructure.com>

Jenkinsfile

@@ -180,6 +180,9 @@ pipeline {
     CASSANDRA_IMAGE_TAG=imageTag.cassandra()
     DYNAMODB_IMAGE_TAG=imageTag.dynamodb()
     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()
   }
 
   stages {