Change gradle lockfile generation script to use --config-only flag (#132967)

The `generate_gradle_lockfiles.dart` script was generating the gradle wrapper by building a flavor that didn't exist. In the time since the script was written, the `--config-only` flag was created and should be used instead.

Context https://github.com/flutter/flutter/pull/132406#discussion_r1300352602

dev/tools/bin/generate_gradle_lockfiles.dart

@@ -86,13 +86,11 @@ void main(List<String> arguments) {
 
     // Verify that the Gradlew wrapper exists.
     final File gradleWrapper = androidDirectory.childFile('gradlew');
-    // Generate Gradle wrapper if it doesn't exists.
-    // This logic is embedded within the Flutter tool.
-    // To generate the wrapper, build a flavor that doesn't exist.
+    // Generate Gradle wrapper if it doesn't exist.
     if (!gradleWrapper.existsSync()) {
       Process.runSync(
         'flutter',
-        <String>['build', 'apk', '--debug', '--flavor=does-not-exist'],
+        <String>['build', 'apk', '--config-only'],
         workingDirectory: appDirectory,
       );
     }