Ensure that command wrapping only runs on Windows

[nvmkuruc]

Description of Change(s)

The shebang.py script has two modes, one which does substitution of \pxrpythonsubst, the other which generates a wrapper command for Windows. The mode is argument dependent. However, if cmake fails to set PXR_PYTHON_SHEBANG, the wrong mode can get executed and a build with errors will silently be installed.

This change separates the unrelated windows only components out of shebang.py into a sibling winpycmd.py script. It also adds a warning to winpycmd to alert users that it's being run on a non-Windows platform.

With this fix, the build will now fail (instead of silently installing a build with errors) when PXR_PYTHON_SHEBANG cannot be correctly set.

Fixes Issue(s)

• cmake/macros/shebang.py runs the wrong code path if PXR_PYTHON_SHEBANG fails to be set to the python executable #3047

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9591

[jesschimein]

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[azure-pipelines]

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