s/vmailUIDStart/vmailUID/g

The name vmailUIDStart is not consistent with how it is being used (as the UID
of the vmail user).

tests/intern.nix

@@ -35,7 +35,7 @@ import <nixpkgs/nixos/tests/make-test.nix> {
           };
 
           vmailGroupName = "vmail";
-          vmailUIDStart = 5000;
+          vmailUID = 5000;
         };
     };