Update cmd/CMakeLists to conform with all other gz libraries

[azeey]

🦟 Bug fix

Summary

The updated cmake file is now similar to other gz libraries, eg. https://github.com/gazebosim/gz-transport/blob/9ba73b74f6ba9717d33429ebf93ef1500c7958f4/src/cmd/CMakeLists.txt

An outcome of updating the cmake files is that it fixes the library path obtained by cmdgui.rb such that it points to the .so file contained in the non-dev debian package.

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.

[jennuine]

ign_TEST is failing

/github/workspace/src/ign_TEST.cc:98: Failure
  Expected: (output.find("TopicEcho")) != (std::string::npos), actual: 18446744073709551615 vs 18446744073709551615
  Library error: [/github/workspace/lib/libignition-gui3.so.3.11.2] not found.
  
  /github/workspace/src/ign_TEST.cc:99: Failure
  Expected: (output.find("Publisher")) != (std::string::npos), actual: 18[4467](https://github.com/gazebosim/gz-gui/runs/8026238453?check_suite_focus=true#step:4:4475)44073709551615 vs 18446744073709551615
  Library error: [/github/workspace/lib/libignition-gui3.so.3.11.2] not found.

[azeey]

ign_TEST is failing

/github/workspace/src/ign_TEST.cc:98: Failure
  Expected: (output.find("TopicEcho")) != (std::string::npos), actual: 18446744073709551615 vs 18446744073709551615
  Library error: [/github/workspace/lib/libignition-gui3.so.3.11.2] not found.
  
  /github/workspace/src/ign_TEST.cc:99: Failure
  Expected: (output.find("Publisher")) != (std::string::npos), actual: 18[4467](https://github.com/gazebosim/gz-gui/runs/8026238453?check_suite_focus=true#step:4:4475)44073709551615 vs 18446744073709551615
  Library error: [/github/workspace/lib/libignition-gui3.so.3.11.2] not found.

Fixed failing test in 4c33a15. I missed the corresponding changes in conf/CMakeLIsts.txt

[codecov]

Codecov Report

Merging #478 (4c33a15) into ign-gui3 (d43ea6a) will not change coverage.
The diff coverage is n/a.

❗ Current head 4c33a15 differs from pull request most recent head 18082b7. Consider uploading reports for the commit 18082b7 to get more accurate results

@@            Coverage Diff            @@
##           ign-gui3     #478   +/-   ##
=========================================
  Coverage     73.51%   73.51%           
=========================================
  Files            30       30           
  Lines          3270     3270           
=========================================
  Hits           2404     2404           
  Misses          866      866           

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[chapulina]

Windows isn't happy

``
CMake Error in src/cmd/CMakeLists.txt:
Evaluation file to be written multiple times with different content. This
is generally caused by the content evaluating the configuration type,
language, or location of object files:

C:/Jenkins/workspace/ign_gui-pr-win/ws/build/ignition-gui3/test/lib/ruby/ignition/cmdgui3.rb

[azeey]

I disabled the cmake generation for cmd in 18082b7 since UNIT_ign_TEST was disabled on windows anyway. In addition, this follows the pattern in ign-transport and ign-msgs.

[azeey]

INTEGRATION_scene3d failed on Github Actions for Bionic, but I see that other PRs have been merged with the same failure, so I'll go ahead and merge this.