only trigger cystalball-map build from main-postmerge

we are seeing some poc builds that replicate the main-postmerge
build triggering the crystalball-map build. let's make sure
only the main-postmerge build triggers crytalball-map.

Change-Id: I13c62838577b67ff199a4118b073c6d700dce837
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/346105
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Michael Hargiss <mhargiss@instructure.com>
QA-Review: James Butters <jbutters@instructure.com>
Product-Review: James Butters <jbutters@instructure.com>
Build-Review: James Butters <jbutters@instructure.com>

Jenkinsfile

@@ -631,7 +631,8 @@ pipeline {
                     ])
 
                   // Trigger Crystalball map build if spec files were added or removed, will not vote on builds.
-                  if (configuration.isChangeMerged() && filesChangedStage.hasNewDeletedSpecFiles(buildConfig)) {
+                  // Only trigger for main-postmerge job.
+                  if (env.JOB_NAME == "Canvas/main-postmerge" && configuration.isChangeMerged() && filesChangedStage.hasNewDeletedSpecFiles(buildConfig)) {
                     build(wait: false, job: 'Canvas/helpers/crystalball-map')
                   }