pmb.helpers.repo.update: support PMB_APK_FORCE_MISSING_REPOSITORIES

Do not abort if one or more APKINDEX files cannot be downloaded, if PMB_APK_FORCE_MISSING_REPOSITORIES is set. This is needed when bootstrapping new stable branches. The same environment variable is already used in pmbootstrap code to pass --force-missing-repositories to apk in pmb.chroot.apk.install_run_apk().