default.nix: ignore some files/dirs when importing source

fixes #35

Author: Mic92

default.nix

@@ -1,7 +1,12 @@
 with import <nixpkgs> {};
 stdenv.mkDerivation {
   name = "nixos-shell";
-  src = ./.;
+  src = builtins.filterSource
+    (path: type: baseNameOf path != "nixos.qcow2" &&
+                 baseNameOf path != ".git" &&
+                 baseNameOf path != ".direnv" &&
+                 baseNameOf path != "result"
+    ) ./.;
   buildInputs = [ bash ];
   preConfigure = ''
     export PREFIX=$out