primeCache(): Barf if builds are needed but not allowed
primeCache(): Barf if builds are needed but not allowed

Fixes #2753.

ehmry/nix,ehmry/nix,vcunat/nix,NixOS/nix,ehmry/nix,dtzWill/nix,dtzWill/nix,rycee/nix,pikajude/nix,rycee/nix,vcunat/nix,NixOS/nix,NixOS/nix,NixOS/nix,pikajude/nix,pikajude/nix,vcunat/nix,dtzWill/nix,vcunat/nix,rycee/nix,ehmry/nix,ehmry/nix,ehmry/nix,NixOS/nix,dtzWill/nix,dtzWill/nix,rycee/nix,rycee/nix