Currently used alpine mirror is outdated (dl-2.alpinelinux.org)

https://mirrors.alpinelinux.org/#mirror3 currently shows that dl-2.alpinelinux.org is outdated by 4 days. I'll switch to a different one in bpo, and pmaports CI. (Note that we don't use dl-cdn there, so we have a more predictable result - if a package builds in CI it should also build in bpo, that might not be the case if a CDN infront of the mirror is outdated.)