This modifies apk cache for indexes to be automatically refreshed
periodically without explicit 'update' or '--update-cache' usage.

The default is to do if-modified-since request if the local copy
is older than 4 hours. This age can be changed with --cache-max-age.
Using --update-cache will change this age to 60 seconds to make
sure the cached copy is relatively new. The small age is in order
to try to avoid downloading indexes second time when apk-tools is
upgraded and apk re-execs after self-upgrade.

Accordingly using explicitly 'apk update' will now enforce
--force-refresh and request the very latest index by requesting
any potential http proxy to do refresh too.