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 gazebosim/gz-tools#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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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

[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 #1389