buildmaster: add static apk utility
We need a relatively recent `apk` that can sign package indexes. You can
find more details in the referenced issue below. However, it has not
been released yet and is not available as a prebuilt download.
Therefore, until it becomes available, let's build a static version and
use that in the meantime.
Fixes: a94d4e15fdc1 ("add APK signing logic")
References: https://gitlab.alpinelinux.org/alpine/apk-tools/-/issues/11008
Signed-off-by: Petr Štetiar <ynezz@true.cz>