Fix runner jobs statistics when incorrect intervalstyle is set

What does this MR do and why?

Fix runner jobs statistics when incorrect intervalstyle is set

ActiveRecord sets ISO8601 as the default style But something in our environment messes it up

And in general, it's a bad idea to rely on the connection variables

So we're simply replacing this with

EXTRACT('EPOCH'

See more in #429227 (closed)

The fix is inspired by Use EXTRACT(EPOCH) when calculating durations (!75505 - merged)

How to set up and validate locally

I relied on tests here

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