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()
.