fix regex (#88976)

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