configs: Use "arm64-ubuntu-20.04-boot" workload for example

The ARM Ubuntu Boot example was using 18.04. This commit updates this
example script to use the "arm64-ubuntu-20.04-boot" workload, added
here:
https://gem5-review.googlesource.com/c/public/gem5-resources/+/62662

Change-Id: I9cee16f739a5fa9281041fde242b5cd37e5be20b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/62665
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed