Allow using alternative pip installers in installation scripts

[yuxuan-z19]

Currently, installation scripts always invoke pip via sys.executable -m pip in subprocesses.
This PR introduces get_pip_cmd(), which:

• Uses the PIP_MODULE environment variable to override the installer if set.

  PIP_MODULE="uv pip" python install.py

• Defaults to sys.executable -m pip otherwise.

This improves flexibility and compatibility with environments that use alternative pip executables. (See install.log for an example with uv.)

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[meta-cla]

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[yuxuan-z19]

Removed the uv fallback, and the installer now only honors PIP_MODULE when provided and otherwise falls back to sys.executable -m pip, as discussed in meta-pytorch/tritonbench#672 (comment)