nix: check config via hakurei

This is unfortunately the only feasible way of doing this in nix.

Signed-off-by: Ophestra <cat@gensokyo.uk>

test/sandbox/configuration.nix

@@ -78,6 +78,7 @@ in
 
     commonPaths = [
       {
+        type = "bind";
         src = "/var/cache";
         write = true;
       }