Hello there!
First off, I was not able to answer to the mail directly since I just recently subscribed to this mailing list and do not have a google account.
But I saw the the mail with the subject Qubes 4.1 Archlinux by Jon Will posted on Saturday, August 15, 2020 at 11:47:02 PM UTC
Maybe there is someone on this list, willing to write Jon a little note so that he can find this mail, if he doesn't follow along with every post.
My response:
It seems that the build process of vmm-xen fails. A few days ago, I also tried building an archlinux template in my Fedora 31 VM on a R4.0 system and encountered a similar issue.
I haven't had time since to try the fixed testing repos mentioned in the issue, but it may assist you with your problem.
Greetings,
Ni!