[cloud functions] Use const constructor for environment variable

Change-Id: Ib8ff395657a9e0c69d9da9933ce7da0602f4f19b
Reviewed-on: https://dart-review.googlesource.com/c/dart_ci/+/156183
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Karl Klose <karlklose@google.com>
diff --git a/functions/node/commits_cache.dart b/functions/node/commits_cache.dart
index ae5b634..6de65f8 100644
--- a/functions/node/commits_cache.dart
+++ b/functions/node/commits_cache.dart
@@ -99,7 +99,7 @@
 
   Future<String> get branchName async {
     if (await firestore.isStaging()) {
-      return String.fromEnvironment('BRANCH') ?? 'master';
+      return const String.fromEnvironment('BRANCH') ?? 'master';
     }
     return 'master';
   }