Fixing a typo in the 'test' toplevel attribute.

pkgs/default.nix

@@ -108,7 +108,7 @@ let
       name = "testEnv";
 
       #required to be able to build the drv.
-      installPhase = "mkdir $$out";
+      installPhase = "mkdir $out";
       unpackPhase = "true";
 
       __noChroot = true;