Change error message when enumeration type param is invariant

[mpilquist]

Minor thing I noticed when working with enumerations:

scala> enum Lst[A]:
     |   case Nil
     |   case Cons(head: A, tail: Lst[A])
     |
2 |  case Nil
  |  ^^^^^^^^
  |  cannot determine type argument for enum parent class Lst,
  |  type parameter type A is non variant

This is the only occurrence of "non variant" in any compiler error messages. PR changes this to "invariant".