top-level: Close over fewer arguments for stdenv stages

This makes the flow of data easier to understand. There's little downside
because the args in question are already inspected by the stdenvs.

cross-compiling in particular is simpler because we don't need to worry
about overriding the config closed over by `allPackages`.

pkgs/stdenv/linux/default.nix

@@ -106,7 +106,7 @@ rec {
       };
 
       thisPkgs = allPackages {
-        inherit system platform;
+        inherit system platform config;
         allowCustomOverrides = false;
         stdenv = thisStdenv;
       };