nixos/exim: Make queue runner interval configurable and reduce it to 5m

Exim spawns a new queue runner every n minutes as configured by the
argument to -q; up to queue_run_max can be active at the same time.
Spawning a queue runner only every 30 mins means that a message that
failed delivery on the first attempt (e.g. due to greylisting) will only
be retried 30 minutes later.

A queue runner will immediately exit if the queue is empty, so it is
more a function on how quickly Exim will scale to mail load and how
quickly it will retry than something that is taxing on an otherwise
empty system.

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