Restore CXX_STANDARD export from core library

[scpeters]

🦟 Bug fix

Restore CXX_STANDARD export in core library that was removed in #369 and also tests gazebosim/gz-cmake#273

Summary

Similar to gazebosim/sdformat#1072, this is mainly testing gazebosim/gz-cmake#273 using the ci_matching_branch/ trickery with osrf/homebrew-simulation@406b74a and gazebo-tooling/gazebodistro@b427433, but it does make a small improvement of its own, restoring the CXX_STANDARD export.

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 #377 (fb25d47) into main (5758a45) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #377   +/-   ##
=======================================
  Coverage   76.77%   76.77%           
=======================================
  Files         128      128           
  Lines        5628     5628           
=======================================
  Hits         4321     4321           
  Misses       1307     1307           

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[ahcorde]

@osrf-jenkins run tests please!