log: take into account env vars on log.cfg call

[TarantoolBot]

Related dev. issue(s): tarantool/tarantool#6011, tarantool/tarantool#7461

Product: Tarantool
Since: 2.11
Root document: https://www.tarantool.io/en/doc/latest/reference/configuration/#logging, https://www.tarantool.io/en/doc/latest/reference/reference_lua/log/#log-module
SME: @ nshy

Details

If env vars like TT_LOG, TT_LOG_LEVEL etc that specify log options are set
then log.cfg() take them into account if called before box.cfg().
Requested by @ nshy in tarantool/tarantool@eb0a4c9.