Set BLAKE2s key only via CMake cache variable

[PatrickKa]

Description

For security reasons, we obviously can't put the value of our key in the source code. I think the solution suggested by Jakob is the easiest one: Make a CMake cache variable for the key, that has no default value and must be set on the command line or via a preset. Of course, it can only be in the CMakeUserPresets.json otherwise the key would end up in the public repository again.