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 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 87.73%. Comparing base (eac2e69) to head (ddc0444). Report is 11 commits behind head on gz-transport13.

Additional details and impacted files

```diff @@ 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 ```

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[azeey]

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