pkgs/nix-direnv: Use explicit paths to nixpkgs utils.

We need to make sure that nix-direnv is explicitly using utils from
nixpkgs, since the operating system versions of the utils may not play
nicely (e.g. BSD-grep on macOS won't work w/ nix-direnv).

See also: nix-community/nix-direnv#3

pkgs/nix-direnv/default.nix

@@ -1,4 +1,4 @@
-{ fetchFromGitHub, stdenv }:
+{ bash, fetchFromGitHub, gnugrep, stdenv }:
 
 stdenv.mkDerivation {
   name = "nix-direnv";
@@ -10,8 +10,13 @@ stdenv.mkDerivation {
     sha256 = "067493hbsij59bvaqi38iybacqbzwx876dvdm651b5mn3zs3h42c";
   };
 
-  phases = [ "unpackPhase" "installPhase" ];
+  phases = [ "unpackPhase" "patchPhase" "installPhase" ];
 
+  prePatch = ''
+    substituteInPlace direnvrc --replace "/usr/bin/env bash" "${bash}/bin/bash"
+    substituteInPlace direnvrc --replace "grep" "${gnugrep}/bin/grep"
+  '';
+
   installPhase = ''
     mkdir -p $out/share/nix-direnv
     cp -rv ./* $out/share/nix-direnv