Respect GZ_PYTHON_INSTALL_PATH if passed as an argument

[j-rivero]

🦟 Bug fix

Summary

The PR adds the ability to set the GZ_PYTHON_INSTALL_PATH from CMake arguments by no overriding it if it is set. This is useful for paths injected by the different package managers and distributions.

One use case is for the ROS vendor packages where the Python path is controlled by an ament_python helper call.

Easier to see https://github.com/gazebosim/sdformat/pull/1586/files?w=1

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
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[scpeters]

macos workflow failure is unrelated: #1587

[scpeters]

macos workflow failure is unrelated: #1587

workaround in #1588

[scpeters]

looks good; we probably want to do this for gz-math and other packages as well

[scpeters]

@j-rivero do we still want this change even though we didn't use it for the vendor packages?

[j-rivero]

@j-rivero do we still want this change even though we didn't use it for the vendor packages?

I think that could be useful for other packagers yes, let's get it now that we did all the work. Thanks guys.