Add new GZ_GUI_RESOURCE_PATH to help message

[mabelzhang]

🎉 New feature

Trivial followup to gazebosim/gz-gui#386

Summary

Trivial PR.

Test it

Check the new var is printed:

$ ign gazebo --help
...
  GZ_GUI_RESOURCE_PATH    Colon separated paths used to locate GUI      
 resources such as configuration files.
...

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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

Merging #1470 (3728963) into ign-gazebo3 (5994f6d) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-gazebo3    #1470   +/-   ##
============================================
  Coverage        34.67%   34.67%           
============================================
  Files               31       31           
  Lines             1993     1993           
============================================
  Hits               691      691           
  Misses            1302     1302           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5994f6d...3728963. Read the comment docs.