Kill all gitea processes before air build (#30477)

Kill all gitea processes before air build (#30477)

So it happened to me multiple times that air leaves zombie processes
after termination. I think ultimately it's some kind of bug in air, but
we can work around.

The change in the delay is unrelated to the zombie processes but seems
to help a bit with duplicate changes resulting in duplicate `make
generate` as seen here:

<img width="424" alt="Screenshot 2024-04-14 at 17 05 47"
src="https://github.com/go-gitea/gitea/assets/115237/6dd1d787-6be3-4fb2-8b0b-cd711c281793">

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Co-authored-by: delvh <dev.lh@web.de>