nix-direnv: fix NIX_BIN_PREFIX substitution in built direnvrc

* Reason: statically set `${NIX_BIN_PREFIX}` within generated
  `direnvrc`.
* Before it only replaced the `-z ${NIX_BIN_PREFIX:-}` check
  which therefore never was `true`. So, also `NIX_BIN_PREFIX` never got
  set such that its usage later always failed execution with
  `NIX_BIN_PREFIX: unbound variable`. With the fix, the line containing
  the check will no longer be replaced.
* Solves https://github.com/nix-community/nix-direnv/issues/70
* See also discussion on #114622

Signed-off-by: Andreas Schmid <service@aaschmid.de>

pkgs/tools/misc/nix-direnv/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, gnugrep, nix }:
+{ lib, stdenv, fetchFromGitHub, gnugrep, nix, nixFlakes }:
 
 stdenv.mkDerivation rec {
   pname = "nix-direnv";
@@ -14,9 +14,8 @@ stdenv.mkDerivation rec {
   # Substitute instead of wrapping because the resulting file is
   # getting sourced, not executed:
   postPatch = ''
-    substituteInPlace direnvrc \
-      --replace "\''${NIX_BIN_PREFIX:-}" "\''${NIX_BIN_PREFIX:-${nix}/bin/}" \
-      --replace "grep" "${gnugrep}/bin/grep"
+    sed -i "1a NIX_BIN_PREFIX=${nixFlakes}/bin/" direnvrc
+    substituteInPlace direnvrc --replace "grep" "${gnugrep}/bin/grep"
   '';
 
   installPhase = ''