apk: Always operate on package cache directory

apk: Always operate on package cache directory

apk does not use any subdirectories under /var/cache/apk to store
packages, which means that our usual tricks to mount package cache
directories from the package cache directory and repository metadata
from the metadata cache directory into the expected locations don't work.

There might be a way to get this to work with overlayfs but this would
be a very complex change. Instead, let's just disable repository metadata
caching for apk and always use the package cache directory for everything.