package: use /dev/null for apk --repositories-file

package: use /dev/null for apk --repositories-file

In preparation for APK version bump, use /dev/null instead of /dev/zero
for --repositories-file to mute an error in recent APK files.

New APK version use modern istream logic that are more sensible to the
kind of file passed and /dev/null is required to correctly handle an
empty repository file.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>