util-gem5art: Fix incorrect type of size in `createArtifact`

The typing here was `int` but had a default value of `None`. The correct
type is therefore `Optional[int]`.

Change-Id: Ibaf63151196b15f68e643fa5c1b290439d6618c8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/66371
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>