Only set branch name in post-submit runs.

Author: yifanyang

buildSrc/src/main/groovy/com/google/firebase/gradle/plugins/measurement/MetricsReportUploader.java

@@ -38,11 +38,10 @@ public static void upload(Project project, String report) {
     String baseCommit = System.getenv("PULL_BASE_SHA");
     String headCommit = System.getenv("PULL_PULL_SHA");
     String baseRef = System.getenv("PULL_BASE_REF");
-    String pullRef = System.getenv("PULL_REFS");
     String pullRequest = System.getenv("PULL_NUMBER");
 
     String commit = headCommit != null && !headCommit.isEmpty() ? headCommit : baseCommit;
-    String branch = pullRef != null && !pullRef.isEmpty() ? pullRef : baseRef;
+    String branch = pullRequest != null && !pullRequest.isEmpty() ? "" : baseRef;
 
     post(project, report, owner, repo, commit, branch, baseCommit, pullRequest);
   }