Fix installation of Ign*.cmake modules on newer versions of CMake

[azeey]

🦟 Bug fix

Summary

Previously, we expected install(CODE ...), to run before install(FILES ...) since that is the order they appear in the CMakeLists.txt file. install(CODE ...) was responsible for creating a copy or symlink of Gz*.cmake files to Ign*.cmake
and putting them in a temporary location. install(FILES ...) would then copy those files to the final install destination. However, it appears that on CMake 3.29.2 (tested on windows), we can no longer rely on the order they appear in the code. From my local testing, install(CODE ...) ran after install(FILES ...).

This PR uses add_custom_command to generate the Ign*.cmake files which avoids any reliance on the order of the install calls.

This was caught during the ROS 2 tutorial party (see osrf/ros2_test_cases#1526)

cc @clalancette

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)
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[clalancette]

This looks like a good fix to me. Thanks @azeey !