Using Pathname to check for absolute path in ruby script

[sauk2]

🦟 Bug fix

Part of gazebosim/gz-tools#7

Summary

As suggested in this comment by @scpeters, porting the fix from gazebosim/gz-plugin#131 for determination of absolute path in ruby script.

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)
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[scpeters]

homebrew build is failing due to osrf/homebrew-simulation#2834

[scpeters]

I experimented with enabling windows tests in d37b501, but I saw a lot of failed to create subprocess errors, so let's not block on that

• https://build.osrfoundation.org/job/gz_transport-14-win/22/

[azeey]

The failing tests are due to osrf/homebrew-simulation#2834 and not related to this change.