fetchurl: Make it overridable.

This provides a workaround for #66499, because this way
#66499 (comment)

    self: super: {
      fetchurl = super.fetchurl.override (old: {
        curl = old.curl.override {
          gssSupport = false;
          # ...
        };
      });
    }

works as expected.

pkgs/top-level/all-packages.nix

@@ -273,7 +273,7 @@ in
   fetchhg = callPackage ../build-support/fetchhg { };
 
   # `fetchurl' downloads a file from the network.
-  fetchurl = import ../build-support/fetchurl {
+  fetchurl = makeOverridable (import ../build-support/fetchurl) {
     inherit lib stdenvNoCC;
     curl = buildPackages.curl.override rec {
       # break dependency cycles