config: switch alpine mirror to dl-cdn

dl-4 is outdated for armhf edge, and currently one package is stuck
because of that. We have been using dl-cdn for pmaports.git CI for some
time now and it seems to be reliable, so let's use that again.

Related: https://builds.sr.ht/~postmarketos/job/600267
Related: https://gitlab.alpinelinux.org/alpine/infra/infra/-/issues/10728
Related: https://gitlab.postmarketos.org/postmarketos/build.postmarketos.org/-/issues/101

Trivial but important fix to unblock bpo, merging after CI passes.