cue/errors: make compile again with Go 1.12

Change-Id: I7f18c140ddba8dedbb9584c95021163c48ddbcdd
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4947
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/errors/errors.go

@@ -474,7 +474,7 @@ func writeErr(w io.Writer, err Error) {
 	}
 
 	for {
-		u := errors.Unwrap(err)
+		u := xerrors.Unwrap(err)
 
 		printed := false
 		msg, args := err.Msg()