sharedown: unbreak build

This is essentially a partial revert of
c271196d42 which was a successful attempt
at unbreaking this build back in November 2023.

That workaround, however, no longer, well.. works.

Instead I figured out what was wrong with the initial version and fixed
that.
Hopefully this fix lasts longer than the one before.

The important bit that was missing is that `export npm_config_nodedir`
line.

The code in question is copied from
https://nixos.org/manual/nixpkgs/unstable/#javascript-yarn2nix-pitfalls
d176767c02/doc/languages-frameworks/javascript.section.md (pitfalls-javascript-yarn2nix-pitfalls))

I suggest following the link for even more links as to why some commands
from there are the way they are.

pkgs/tools/misc/sharedown/default.nix

@@ -6,7 +6,7 @@
 , libsecret
 , python3
 , pkg-config
-, nodePackages
+, nodejs
 , electron
 , makeWrapper
 , makeDesktopItem
@@ -63,7 +63,6 @@ stdenvNoCC.mkDerivation rec {
             nativeBuildInputs = [
               python3
               pkg-config
-              nodePackages.node-gyp
             ];
             buildInputs = [
               libsecret
@@ -76,6 +75,15 @@ stdenvNoCC.mkDerivation rec {
           };
         };
 
+        # needed for node-gyp, copied from https://nixos.org/manual/nixpkgs/unstable/#javascript-yarn2nix-pitfalls
+        # permalink: https://github.com/NixOS/nixpkgs/blob/d176767c02cb2a048e766215078c3d231e666091/doc/languages-frameworks/javascript.section.md#pitfalls-javascript-yarn2nix-pitfalls
+        preBuild = ''
+          mkdir -p $HOME/.node-gyp/${nodejs.version}
+          echo 9 > $HOME/.node-gyp/${nodejs.version}/installVersion
+          ln -sfv ${nodejs}/include $HOME/.node-gyp/${nodejs.version}
+          export npm_config_nodedir=${nodejs}
+        '';
+
         packageJSON = "${src}/package.json";
         yarnLock = ./yarn.lock;