Use relative install paths for plugin shared libraries and gz-tools data

[azeey]

🦟 Bug fix

Summary

Fixes an error when building https://github.com/gazebo-release/gz_gui_vendor/ in the ROS buildfarm.

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 70.09%. Comparing base (b252bd0) to head (c89e734).

Additional details and impacted files

@@           Coverage Diff            @@
##           gz-gui8     #614   +/-   ##
========================================
  Coverage    70.09%   70.09%           
========================================
  Files           39       39           
  Lines         5363     5363           
========================================
  Hits          3759     3759           
  Misses        1604     1604           

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[azeey]

This can be tested by going into the build directory and running

cmake -DCMAKE_ERROR_ON_ABSOLUTE_INSTALL_DESTINATION=ON -P cmake_install.cmake

(Make sure to have the latest version of gz-cmake3)