nix repl: Don't build in a child process
nix repl: Don't build in a child process

Fixes #5356. This is a bit risky due to interrupts, but we have to
deal with those anyway (#5353).

NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix