fix recompute --reproducible run on a VURL key

This avoids "Cannot generate a key for backend VURL", and makes it use
the usual hashing backend.

Command/AddComputed.hs

@@ -52,6 +52,7 @@ optParser desc = AddComputedOptions
 	<*> parseReproducible
 
 newtype Reproducible = Reproducible { isReproducible :: Bool }
+	deriving (Show, Eq)
 
 parseReproducible :: Parser (Maybe Reproducible)
 parseReproducible = r <|> unr