Thanks for the reply! I didn’t actually know I could use unstable.legacyPackages.${system}.callPackage like this! Of course system was not defined, so I replaced it with x86_64-linux for my use-case.
However, when I tried nixos-rebuild switch, cmake began complaining. I filed this bug-report afterwards. This doesn’t seem to be related to my configuration-issue, but I can’t completely confirm that it’s working now, because of cmake being broken atm.