Fix gz plugin paths in windows

[iche033]

🦟 Bug fix

Summary

Replaced IGNITION_GAZEBO_PLUGIN_PATH with GZ_SIM_PLUGIN_PATH for windows for creating unversioned libraries.

I noticed that the IGNITION_GAZEBO_PLUGIN_PATH was completely removed in garden without being deprecated so I added it in config.hh

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

Merging #1899 (90e63c4) into gz-sim7 (4380ab1) will increase coverage by 0.01%.
The diff coverage is 57.14%.

@@             Coverage Diff             @@
##           gz-sim7    #1899      +/-   ##
===========================================
+ Coverage    64.56%   64.58%   +0.01%     
===========================================
  Files          344      344              
  Lines        27578    27590      +12     
===========================================
+ Hits         17807    17818      +11     
- Misses        9771     9772       +1     

Impacted Files	Coverage Δ
...int_position_controller/JointPositionController.cc	74.13% <57.14%> (+0.68%)	⬆️
src/systems/physics/Physics.cc	66.34% <0.00%> (+0.06%)	⬆️

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[iche033]

The Failed to load system plugin error messages are now gone in the full console log in windows build:(https://build.osrfoundation.org/job/ign_gazebo-pr-win/5500/consoleFull.
As an example, the error messages were present in: https://build.osrfoundation.org/job/ign_gazebo-pr-win/5494/consoleFull