pmb/aportgen: lookup package in APKINDEX with correct arch (MR 2493)

This can otherwise fail if the package was only built for some
architectures so far, but not for the architecture that is being generated.

E.g. right now musl-1.2.5-r6 was built already for x86_64 but not aarch64,
which causes errors like the following when running "pmbootstrap aportgen
musl-x86_64" on aarch64:

>>> musl-x86_64: Fetching musl-1.2.5-r5-x86_64-edge.apk::http://dl-4.alpinelinux.org/alpine//edge/main/x86_64/musl-1.2.5-r5.apk
Connecting to dl-4.alpinelinux.org (147.75.40.42:80)
Connecting to dl-4.alpinelinux.org (147.75.40.42:443)
wget: server returned error: HTTP/1.1 404 Not Found
>>> ERROR: musl-x86_64: checksum failed