ci: add a combined job parameter to choose E2E git ref

ci/Jenkinsfile.combined

@@ -28,6 +28,11 @@ pipeline {
       description: 'Trigger publishing of build results to GitHub.',
       defaultValue: getPublishDefault(params.PUBLISH),
     )
+    string(
+      name: 'E2E_GIT_REF',
+      description: 'GIT_REF for E2E repo.',
+      defaultValue: params.E2E_GIT_REF ?: 'master',
+    )
   }
 
   stages {
@@ -63,6 +68,7 @@ pipeline {
             BUILD_SOURCE:       linux_x86_64.fullProjectName,
             TESTRAIL_RUN_NAME:  utils.pkgFilename(),
             TEST_SCOPE_FLAG:    utils.isReleaseBuild() ? '-m=critical' : '',
+            GIT_REF:            params.E2E_GIT_REF,
           ]),
         )
       } }