rkt: Don't download stage1 image during build.

Second attempt to resolve this issue. Copies stage1 image into expected
place manually. This has been improved in rkt master where there is a
configure option for specifying the location of this file. Can update
when next stable rkt is released.

pkgs/applications/virtualization/rkt/default.nix

@@ -26,7 +26,12 @@ in stdenv.mkDerivation rec {
     ./autogen.sh
   '';
 
-  configureFlags = "--with-stage1-image-path=${stage1image}";
+  preBuild = ''
+    # hack to avoid downloading image during build, this has been
+    # improved in rkt master
+    mkdir -p build-rkt-0.8.0/tmp/usr_from_coreos
+    cp -v ${stage1image} build-rkt-0.8.0/tmp/usr_from_coreos/pxe.img
+  '';
 
   installPhase = ''
     mkdir -p $out/bin