nix-daemon: add variable to disable fork safety
nix-daemon: add variable to disable fork safety

Since macOS 10.14 this has become an error, causing problems if the
nix-daemon loads nix during substitution (this is a forked process).

Workaround for #2523.

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