only set ccbin flag if not provided by user (#47404)
Summary:
Avoid nvcc error if the user specifies c compiler (as pointed out in https://github.com/pytorch/pytorch/issues/47377)
Fixes https://github.com/pytorch/pytorch/issues/47377
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47404
Reviewed By: ejguan
Differential Revision: D24748833
Pulled By: malfet
fbshipit-source-id: 1a4ad1f851c8854795f7f98e28f479a0ff458a00
