sharedown: unbreak build

fixes the following error message:

```
$ node-gyp rebuild
info This package requires node-gyp, which is not currently installed. Yarn will attempt to automatically install it. If this fails, you can run "yarn global add node-gyp" to manually install it.
[1/4] Resolving packages...
error Failed to auto-install node-gyp. Please run "yarn global add node-gyp" manually. Error: "Couldn't find any versions for \"node-gyp\" that matches \"latest\" in our cache (possible versions are \"\"). This is usually caused by a missing entry in the lockfile, running Yarn without the --offline flag may help fix this issue."
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
```

https://hydra.nixos.org/build/239684452

pkgs/tools/misc/sharedown/default.nix

@@ -6,7 +6,7 @@
 , libsecret
 , python3
 , pkg-config
-, nodejs
+, nodePackages
 , electron
 , makeWrapper
 , makeDesktopItem
@@ -63,6 +63,7 @@ stdenvNoCC.mkDerivation rec {
             nativeBuildInputs = [
               python3
               pkg-config
+              nodePackages.node-gyp
             ];
             buildInputs = [
               libsecret
@@ -75,17 +76,6 @@ stdenvNoCC.mkDerivation rec {
           };
         };
 
-        preBuild = ''
-          # Set up headers for node-gyp, which is needed to build keytar.
-          mkdir -p "$HOME/.cache/node-gyp/${nodejs.version}"
-
-          # Set up version which node-gyp checks in <https://github.com/nodejs/node-gyp/blob/4937722cf597ccd1953628f3d5e2ab5204280051/lib/install.js#L87-L96> against the version in <https://github.com/nodejs/node-gyp/blob/4937722cf597ccd1953628f3d5e2ab5204280051/package.json#L15>.
-          echo 9 > "$HOME/.cache/node-gyp/${nodejs.version}/installVersion"
-
-          # Link node headers so that node-gyp does not try to download them.
-          ln -sfv "${nodejs}/include" "$HOME/.cache/node-gyp/${nodejs.version}"
-        '';
-
         packageJSON = "${src}/package.json";
         yarnLock = ./yarn.lock;