Respect GZ_PYTHON_INSTALL_PATH if passed as an argument (backport #1586)

[mergify]

🦟 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)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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This is an automatic backport of pull request #1586 done by Mergify.