nodejs: use .finalPackage for ease of overriding

Previously, the nodejs `passthru.pkgs` attribute refers to the package
fixed point derived in the current context, without considering
potential overrides to the main nodejs package.

This is fixed here with the `.finalPackage` attribute, which refers to
the final package fixed point, after potential overrides.

For example, previously, after overriding nodejs with `nodejs.override`,
`nodejs.pkgs` would still refer to the original package, instead of the
overridden one. After this fix, `nodejs.pkgs` would be correctly based
on the overridden `nodejs` package.

pkgs/development/web/nodejs/nodejs.nix

@@ -44,7 +44,13 @@ let
       (builtins.attrNames sharedLibDeps);
 
   extraConfigFlags = lib.optionals (!enableNpm) [ "--without-npm" ];
-  self = stdenv.mkDerivation {
+
+  package = stdenv.mkDerivation (finalAttrs:
+  let
+    /** the final package fixed point, after potential overrides */
+    self = finalAttrs.finalPackage;
+  in
+  {
     inherit pname version;
 
     src = fetchurl {
@@ -235,5 +241,5 @@ let
     };
 
     passthru.python = python; # to ensure nodeEnv uses the same version
-  };
-in self
+  });
+in package