Set `NIX_PATH` in `shellHook` of `shell.nix`.

If nix-shell is run interactively on NixOS, in way that has loaded /etc/profile (for example docker exec <container> nix-shell), then /etc/profile will be sourced in the shell that nix-shell starts. That overwrites the NIX_PATH set by shell.nix.

If we instead set NIX_PATH in the the shellHook, that always takes precedence.