Fix gradle patching when using @..

fdroidserver/build.py

@@ -606,7 +606,7 @@ def build_local(app, thisbuild, vcs, build_dir, output_dir, srclib_dir, extlib_d
                         's@compileSdkVersion[ ]*[0-9]*@compileSdkVersion '+level+'@g',
                         'build.gradle'], cwd=gradle_dir)
 
-        for root, dirs, files in os.walk(build_dir):
+        for root, dirs, files in os.walk(gradle_dir):
             for f in files:
                 if f == 'build.gradle':
                     adapt_gradle(os.path.join(root, f))