pmbootstrap aportgen: don't use CDN alpine mirror
Use a deterministic mirror URL instead of CDN for aportgen. Otherwise we may generate a pmaport that wraps an apk from Alpine (e.g. musl-armv7) locally with one up-to-date mirror given by the CDN. But then the build will fail if CDN picks an outdated mirror for CI or BPO.
CI will fail due to 32-bit repos on edge being rebuilt. Creating this MR for reference, as I'm updating the pmaports with this change. We can run CI again and get it reviewed / merged after the rebuild.