main/postmarketos-base: enable apk --cache-pre-download

This allows safer installation of packages, since the upgrade doesn't
fail with a half-installed system if the download fails while installing.

Note that this will only take effect when Alpine upgrades to apk-tools 3.

Closes: postmarketOS/pmaports#3383
See: https://gitlab.alpinelinux.org/alpine/apk-tools/-/issues/10891