fix regex (flutter#88976)

Author: christopherfujino

dev/conductor/lib/repository.dart

@@ -823,7 +823,7 @@ class CiYaml {
  }).toList();
  }
 
- static final RegExp _enabledBranchPattern = RegExp(r'^enabled_branches:');
+ static final RegExp _enabledBranchPattern = RegExp(r'enabled_branches:');
 
  /// Update this .ci.yaml file with the given branch name.
  ///

dev/conductor/test/repository_test.dart

@@ -405,6 +405,8 @@ vars = {
  onRun: () {
  ciYaml.createSync(recursive: true);
  ciYaml.writeAsStringSync('''
+# Friendly note
+
 enabled_branches:
  - master
  - dev
@@ -441,6 +443,8 @@ enabled_branches:
  expect(
  framework.ciYaml.stringContents,
  '''
+# Friendly note
+
 enabled_branches:
  - foo
  - master