Add a minimum CI run duration

[Kobzol]

Sometimes, when someone does a stupid thing and it completely removes all tests from CI, the CI workflows can finish very quickly, and e.g. run no jobs. Bors should have some configurable default to avoid accepting a merge when CI took less than a pre-defined time.

[MarcoIeni]

In case anyone wants to work on this, here's how you can add the field to the config: main...MarcoIeni:bors:min-ci