Use Alpine Stable for CI

Currently, we're using Alpine Edge for the CI here in pmbootstrap. This can be a problem whenever there is some form of disruption on Edge as it can result in our CI malfunctioning which prevents us from merging.

The two missing components are Ruff and Codespell, neither of which are available in Alpine 3.19. Ruff will be available in Alpine 3.20 as it was moved to community during this release cycle: https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/62711, but Codespell is yet to be moved. I raised an issue about this in the aports issue tracker: https://gitlab.alpinelinux.org/alpine/aports/-/issues/16008