Use relative install path for gz tool data

[azeey]

🦟 Bug fix

Summary

Fixes an error when building https://github.com/gazebo-release/gz_transport_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 87.73%. Comparing base (eac2e69) to head (ddc0444).
Report is 11 commits behind head on gz-transport13.

Additional details and impacted files

@@                Coverage Diff                 @@
##           gz-transport13     #492      +/-   ##
==================================================
+ Coverage           87.69%   87.73%   +0.04%     
==================================================
  Files                  59       59              
  Lines                5704     5708       +4     
==================================================
+ Hits                 5002     5008       +6     
+ Misses                702      700       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[azeey]

The homebrew failure is due to a recent change in protobuf and not related to this PR. I'll go ahead and merge.