CMake: Fix running Octopus from installation

Changes to share/CMakeLists.txt in MR 2958 caused these files and directories to be missed from the installation:

CMakeFiles
cmake_install.cmake
dftd3
testsuite
variables
varinfo
varinfo_orig

which break running Octopus from an installation. I've restored the copying of:

dftd3
testsuite
variables
varinfo
varinfo_orig

Thanks for @lang-m for investigating the cause of this problem 🫶🏻. It would be beneficial if you could pull this branch and add a step that tests running octopus from an installation (which is done in docker, but this way would be more transparent).

Also tagging @martin.lueders so he's aware of the fix