Thanks a lot. It took a while since I came back to this question. It worked brilliantly comfortable and with great ease. But ... It produced another incomprehensible message:
PacletInstall::compat: The paclet CodeInspector was successfully installed, but it will not be available in this session because it is not compatible with the currently running version of the Wolfram system.
I did obey the recommendation from github.com to build it with my version of Mathematica.
Same messages with PacletInstall and path as with cmake --install.