package.nix: stop rebuilding aws-sdk-cpp

A friend reported that this is rebuilding, which doesn't make any sense
because nixpkgs also has a lix from nightly that surely has the same
dependency and thus it surely should be built on hydra, right?

Turns out no, since they were overriding the requiredSystemFeatures to
remove big-parallel. Silly, but we can match that and get cache hits.

Change-Id: I4b1c8e850c6c38b03881354e978f41a10592ec95

package.nix

@@ -168,13 +168,19 @@ let
     if aws-sdk-cpp == null then
       null
     else
-      aws-sdk-cpp.override {
+      (aws-sdk-cpp.override {
         apis = [
           "s3"
           "transfer"
         ];
         customMemoryManagement = false;
-      };
+      }).overrideAttrs
+        {
+          # this overrideAttrs is needed to match the configuration of
+          # aws-sdk-cpp in nixpkgs' builds of lix so we don't rebuild this in
+          # practice
+          requiredSystemFeatures = [ ];
+        };
 
   # Reimplementation of Nixpkgs' Meson cross file, with some additions to make
   # it actually work.