Support nixpkgsArgs (args passed to nixpkgs) for release build

Default enabled allowUnfree by default (differs from upstream).

release.nix

@@ -8,6 +8,7 @@
         revCount = 56789;
         shortRev = "gfedcba";
       }
+, nixpkgsArgs ? { config = { allowUnfree = true; inHydra = true; }; }
 , stableBranch ? false
 , supportedSystems ? [ "x86_64-linx" ] }:
 
@@ -31,7 +32,9 @@ in let
 
   # pkgs - Packages sucked in from the given ccpkgs git version.
   #
-  pkgs = import ccpkgsGit { upstream = nixpkgsGit; };
+  pkgs = import ccpkgsGit {
+    upstream = nixpkgsGit;
+  } // nixpkgsArgs;
 
   # version        - Version number (e.g., "17.09")
   # version*Suffix - Version suffix (e.g., "5678.gfedcba")