Replace errors.Cause() with errors.Is()

Dependencies may be switching to use the new `%w` formatting
option to wrap errors; switching to use `errors.Is()` makes
sure that we are still able to unwrap the error and detect the
underlying cause.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

runtime/v1/linux/task.go

@@ -159,7 +159,7 @@ func (t *Task) State(ctx context.Context) (runtime.State, error) {
 		ID: t.id,
 	})
 	if err != nil {
-		if errors.Cause(err) != ttrpc.ErrClosed {
+		if !errors.Is(err, ttrpc.ErrClosed) {
 			return runtime.State{}, errdefs.FromGRPC(err)
 		}
 		return runtime.State{}, errdefs.ErrNotFound