abuild: fix for apk-tools-2.0.x

apk-tools-2.0's apk index will not show the arch so we need to use
tar to find arch if we have old apk. We prefer use apk index since
it will only read first block of file, regardless size, while tar
will read entire file. Reading entire file is slow if file is big.

So we check apk --version and use tar if needed, and apk index if
possible.