Use relative install path for gz tool data

[azeey]

🦟 Bug fix

Summary

Fixes an error when building https://github.com/gazebo-release/gz_plugin_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 98.26%. Comparing base (faadc0d) to head (95b8eeb).

Additional details and impacted files

@@             Coverage Diff             @@
##           gz-plugin2     #140   +/-   ##
===========================================
  Coverage       98.26%   98.26%           
===========================================
  Files              23       23           
  Lines             748      748           
===========================================
  Hits              735      735           
  Misses             13       13           

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