Add $CONDA_ENV/bin to PATH on MacOS (#98522)
This PR explicitly add $CONDA_ENV/bin to MacOS PATH, so that it can always detect and use the correct Python. $CONDA_ENV is always set to the correct value in setup-miniconda https://github.com/pytorch/test-infra/blob/main/.github/actions/setup-miniconda/action.yml#L141
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### <samp>🤖 Generated by Copilot at b4de81a</samp>
This pull request fixes the conda-pip environment mismatch for the macOS build and test workflows by using consistent pip requirements files. It also adds a conditional block to the `.github/workflows/_mac-test-mps.yml` file to enable the test MPS job.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98522
Approved by: https://github.com/malfet
