Use relative install path for gz bash completion data

[azeey]

🦟 Bug fix

Summary

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

The DESTINATION argument of install can take relative paths which is

interpreted relative to the value of the CMAKE_INSTALL_PREFIX variable. The prefix can be relocated at install time using the DESTDIR mechanism explained in the CMAKE_INSTALL_PREFIX variable documentation.

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

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