A small optimization for the personal roboport

[Tinyboss]

When a construction bot in my personal roboport is assigned a job, it should not launch if I am moving toward that job faster than the bot can fly. The way it works now, it's like if you're riding a train to your outpost, and you think, "hey, I should build some more miners there", and so you immediately jump out of the train and start running.

It's a small thing, but it would also be very easy to code, and I think it fits with the spirit of Factorio to do things as intelligently as possible!