pmb.aportgen: use correct arch for gcc aportgen

Previously it always used x84_64 arch for APKINDEX lookup.

As pmb.helpers.repo.alpine_apkindex_path() already has arch parameter, use it to look up correct APKINDEX.

Without this, for example, currently, due to x86_64 Alpine builder is busy building chromium upgrade, it is not possible to generate gcc-aarch64, even though aarch64 builder has finished its work and upgraded gcc -r2 package is available for aarch64, pmbootstrap aportgen gcc-aarch64 returns unexpected:

ERROR: Package 'gcc' has a different version in local checkout of Alpine's aports (10.3.1_git20210424-r2) compared to Alpine's binary package (10.3.1_git20210424-r1)!

This is simply not true! Yes, Alpine's binary package for x86_64 is still -r1, but for aarch64 it should work!