Use GitHub mirrors of mlplatform submodules to unbreak ExecuTorch CI (#6991)

CI jobs have been failing due to availability issues with these submodules. mirrors should unbreak CI.

Differential Revision: [D66255556](https://our.internmc.facebook.com/intern/diff/D66255556/)

ghstack-source-id: 254626070
Pull Request resolved: https://github.com/pytorch/executorch/pull/6989

Co-authored-by: Scott Wolchok <swolchok@fb.com>