nix-direnv: make flakes support optional

`nix-direnv` depends on `nixUnstable` for flakes support, but that
causes it to print a warning unless your user is trusted or the
`experimental-features` setting is enabled. Neither of these are
desirable (note that setting `experimental-features` in nix.conf causes
warnings on `nixStable`).

Since flakes are experimental, this commit makes `nixStable` the default
and optionally allows `nixUnstable` with a new flag `enableFlakes`,
prioritising a good experience for users not using experimental
features.

(cherry picked from commit 03310df843a344215b45b2e8aef11ae9402a40e2)

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

@@ -1,5 +1,15 @@
-{ lib, stdenv, fetchFromGitHub, gnugrep, nixUnstable }:
+{ lib
+, stdenv
+, fetchFromGitHub
+, gnugrep
+, nixStable
+, nixUnstable
+, enableFlakes ? false
+}:
 
+let
+  nix = if enableFlakes then nixUnstable else nixStable;
+in
 stdenv.mkDerivation rec {
   pname = "nix-direnv";
   version = "1.2.6";
@@ -14,7 +24,7 @@ stdenv.mkDerivation rec {
   # Substitute instead of wrapping because the resulting file is
   # getting sourced, not executed:
   postPatch = ''
-    sed -i "1a NIX_BIN_PREFIX=${nixUnstable}/bin/" direnvrc
+    sed -i "1a NIX_BIN_PREFIX=${nix}/bin/" direnvrc
     substituteInPlace direnvrc --replace "grep" "${gnugrep}/bin/grep"
   '';