CI: Switch from `ubuntu-latest-xl` runner image to `ubuntu-20.04-xl`

[hkratz]

The following warning is produced on every CI build (e.g. here):

ubuntu-20.04-xl runner image is now available. The existing ubuntu-latest-xl runner image (based on Ubuntu 18.04 XL) is deprecated and will retire on January 15, 2022. Migrate to ubuntu-20.04-xl instead.

We should preemptively switch earlier so we can fix issues if needed. Not creating a PR as I cannot test it because xl runners are not generally available.

[hkratz]

@rustbot label +T-infra +A-github-actions

[hkratz]

Fixed with #92983.