In pmb.helpers.repo.alpine_apkindex_path(), default to
arch = args.arch_native *before* calling pmb.helpers.repo.update().
Because otherwise update() defaults to all arches instead of
args.arch_native.
This caused the APKINDEX files for all arches to get downloaded, as
pmb.chroot.apk_static.init() calls alpine_apkindex_path() without
setting arch = args.arch_native explicitly.
d7ec81a00e