Raspbian builds were running on Ubuntu 20.04 but with qemu-user-static from Ubuntu 21.10 (impish) due to need for newer features. However, the impish package repos are no longer available as that version reached its EOL. ubuntu-latest still refers to 20.04 on GitHub Actions but 22.04 is already available when explicitly specified. This commit switches the build over to that version and removes the workaround.