setup-apkrepos: Try to fetch the mirrorlist more than one time

Currently, the mirror list is only fetched once, upon entering the setup-apkrepos this part of setup-alpine. Thus, if this (the very first network access that is done, once networking has been setup, right?!) fails, you'll end up without any mirrorlist. I suggest moving the

if [ -z "$MIRRORS" ]; then
       MIRRORS=$(wget -qO- $MIRRORS_URL)
fi

To within the prompting while-loop.