Preserve write permissions when uploading canvaskit to CIPD (flutter/engine#30960)

Related to https://github.com/dart-lang/sdk/issues/48010

engine/src/flutter/lib/web_ui/dev/canvaskit_roller.dart

@@ -56,6 +56,7 @@ data:
     'create',
     '--tag=version:$canvaskitVersion',
     '--pkg-def=cipd.yaml',
+    '--preserve-writable',
     '--json-output=result.json',
   ], workingDirectory: canvaskitDirectory.path);