nix: use upstream GHC for static build

This way we benefit from NixOS' binary cache to deliver GHC for us and
don't need to cache it ourselves.

This will become relevant once we do that for more platforms.

nix/static.nix

@@ -8,17 +8,7 @@ let
   inherit (pkgs) pkgsStatic;
   inherit (pkgsStatic.haskell) lib;
 
-  packagesStatic =
+  packagesStatic = pkgsStatic.haskell.packages.native-bignum."${compiler}";
-    pkgsStatic.haskell.packages."${compiler}".override (old: {
-      ghc = pkgsStatic.pkgsBuildHost.haskell.compiler."${compiler}".override {
-        # Using the bundled libffi generally works better for cross-compiling
-        libffi = null;
-        # Building sphinx fails on some platforms
-        enableDocs = false;
-        # Cross compiling with native bignum works better than with gmp
-        enableNativeBignum = true;
-      };
-    });
 
   makeExecutableStatic = drv: pkgs.lib.pipe drv [
     lib.compose.justStaticExecutables