Fix hanging of flutter run when running on more than one simulator. (#23932)

This stops reading from pipe-to-file log after the build is completed. Attempting to read from pipe-to-file keeps open blocked because nobody is going to write into that pipe.

packages/flutter_tools/lib/src/ios/mac.dart

diff --git a/packages/flutter_tools/lib/src/ios/mac.dart b/packages/flutter_tools/lib/src/ios/mac.dart
index 3ad82ff..a3a2aab 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -441,6 +441,7 @@
         if (line == 'done') {
           buildSubStatus?.stop();
           buildSubStatus = null;
+          return null;
         } else {
           initialBuildStatus.cancel();
           buildSubStatus = logger.startProgress(