Add a default.nix for nix-build

byg/nix/common.nix

@@ -0,0 +1,9 @@
+let
+  sources = import ./sources.nix;
+  pkgs = import sources.nixpkgs {};
+  haskell = pkgs.haskell.packages.ghc910;
+in
+{
+  pkgs = pkgs;
+  haskell = haskell;
+}