Use flutter from in same repo (not path) in generate_gradle_lockfiles.dart (again) (#155794)

Fixes another invocation of `flutter`. Follow up to https://github.com/flutter/flutter/pull/155790/.

Checked that this is the only other one (and also the only use of `exec()` left that is relying on the path and shouldn't be (one other use uses git from the path, which is fine, and the other one uses an absolute path).

dev/tools/bin/generate_gradle_lockfiles.dart

@@ -190,7 +190,7 @@ void main(List<String> arguments) {
     // Generate Gradle wrapper if it doesn't exist.
     if (!gradleWrapper.existsSync()) {
       exec(
-        'flutter',
+        flutterPath,
         <String>['build', 'apk', '--config-only'],
         workingDirectory: appDirectory,
       );