Use relative install paths in CMake

[azeey]

🦟 Bug fix

Summary

This fixes an error when building https://github.com/gazebo-release/sdformat_vendor/ in the ROS buildfarm where CMAKE_STAGING_PREFIX is used

Similar to https://github.com/gazebosim/gz-tools/pull/137

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)
• [ ] 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

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.42%. Comparing base (ff9b3ad) to head (a038b59). Report is 2 commits behind head on sdf14.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## sdf14 #1387 +/- ## ======================================= Coverage 92.42% 92.42% ======================================= Files 134 134 Lines 17751 17751 ======================================= Hits 16406 16406 Misses 1345 1345 ```

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[scpeters]

brew CI failure looks unrelated, I believe caused by update to pybind11:

• pybind11 2.12.0 Homebrew/homebrew-core#167370
• feat: add py::set_error(), use in updated py::exception<> documentation pybind/pybind11#4772

fix in https://github.com/gazebosim/sdformat/pull/1389