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