[cloud functions] Don't use new 'hash' field from unlanded change

Previous change
https://dart-review.googlesource.com/c/dart_ci/+/137982
used this new field that is added in another CL, not landed yet.

Change-Id: I653e8f883e7645fdd14d6d0248d0d561400738bd
Reviewed-on: https://dart-review.googlesource.com/c/dart_ci/+/138640
Reviewed-by: Karl Klose <karlklose@google.com>
diff --git a/functions/node/builder.dart b/functions/node/builder.dart
index c4ec516..da11e17 100644
--- a/functions/node/builder.dart
+++ b/functions/node/builder.dart
@@ -160,7 +160,7 @@
   /// times simultaneously.
   Future<void> getMissingCommits() async {
     final lastCommit = await firestore.getLastCommit();
-    final lastHash = lastCommit[fHash];
+    final lastHash = lastCommit['id'];
     final lastIndex = lastCommit[fIndex];
 
     final logUrl = 'https://dart.googlesource.com/sdk/+log/';