onnxruntime

Use build.py to build in py-win-gpu.yml so parallelization parameters are set (#19578)

### Description
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build.py sets a few parallelization parameters when building. Using
msbuild directly lacks those.


https://github.com/microsoft/onnxruntime/blob/7a5860e4909387448cb51351d3af50933238ba10/tools/ci_build/build.py#L1665-L1669

Changed to use build.py. If there's a concern with that we _could_ set
the parameters in the yaml, but that will be uglier due to duplicating
logic in multiple places.


### Motivation and Context
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- If it fixes an open issue, please link to the issue here. -->