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">

---------

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

.air.toml

@@ -2,9 +2,10 @@ root = "."
 tmp_dir = ".air"
 
 [build]
+pre_cmd = ["killall -9 gitea 2>/dev/null || true"] # kill off potential zombie processes from previous runs
 cmd = "make --no-print-directory backend"
 bin = "gitea"
-delay = 1000
+delay = 2000
 include_ext = ["go", "tmpl"]
 include_file = ["main.go"]
 include_dir = ["cmd", "models", "modules", "options", "routers", "services"]