Fix generation of systems library symlinks in build directory

[peci1]

🦟 Bug fix

Fixes #1152

Summary

As discussed in #1152, the unversioned library symlinks for systems are generated into the root of the build directory, but tests expect them in the lib subdirectory of the build directory. This PR fixes the location.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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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[codecov]

Codecov Report

Merging #1160 (6237a2b) into ign-gazebo3 (9cafc6b) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-gazebo3    #1160   +/-   ##
============================================
  Coverage        77.47%   77.47%           
============================================
  Files              222      222           
  Lines            12769    12769           
============================================
  Hits              9893     9893           
  Misses            2876     2876           

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[peci1]

CI is green except Mac OS. The failures seem unrelated to me.

[chapulina]

Thanks, I verified locally that the build folder is fixed and the install folder remains the same.