I鈥檓 not sure if this is a temporary hiccup or not, but this issue resurfaced for me this evening. I was able to get things running by using the Rackspace mirror:
https://mirror.rackspace.com/mariadb/repo/10.5/ubuntu
See 馃敡 Change MariaDB PPA mirror to Rackspace by retlehs 路 Pull Request #1422 路 roots/trellis 路 GitHub