Don't install CMakeLists.txt files

[scpeters]

🦟 Bug fix

Fixes unstable debbuilds

Summary

The debbuilds from 2.6.0 are unstable due to some CMakeLists.txt files being accidentally installed.

• https://build.osrfoundation.org/job/ign-physics2-debbuilder/140/

This takes an approach similar to gazebosim/gz-common#449 to ensure that they aren't installed.

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 #456 (f2c7154) into ign-physics2 (f35c05d) will not change coverage.
The diff coverage is n/a.

❗ Current head f2c7154 differs from pull request most recent head 12de1ca. Consider uploading reports for the commit 12de1ca to get more accurate results

@@              Coverage Diff              @@
##           ign-physics2     #456   +/-   ##
=============================================
  Coverage         83.15%   83.15%           
=============================================
  Files               108      108           
  Lines              4210     4210           
=============================================
  Hits               3501     3501           
  Misses              709      709           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.