Do not redefine GZ_PYTHON_INSTALL_PATH if it is already defined

[traversaro]

🎉 New feature

Summary

When doing packaging, it is sometimes useful for advanced users to just tell the build system where to install Python files, without any sort of auto-detection. This was already supported elsewhere (see https://github.com/gazebosim/gz-msgs/blob/0472ba0bb5fe39d8a14499155c68746109d9acf7/core/CMakeLists.txt#L69 and https://github.com/gazebosim/gz-msgs/blob/gz-msgs10/cmake/gz_msgs_generate.cmake#L273) but was not supported at the root level.

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Added example and/or tutorial
• [x] Updated documentation (as needed)
• [x] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [x] codecheck passed (See contributing)
• [x] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov[bot]]

Codecov Report

Merging #393 (019129f) into gz-msgs10 (0472ba0) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 019129f differs from pull request most recent head e2cc2ab. Consider uploading reports for the commit e2cc2ab to get more accurate results

@@            Coverage Diff             @@
##           gz-msgs10     #393   +/-   ##
==========================================
  Coverage      97.26%   97.26%           
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  Files             27       27           
  Lines           1170     1170           
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  Hits            1138     1138           
  Misses            32       32