scripts: signall: fix finish function not called when apk adbsign failed

scripts: signall: fix finish function not called when apk adbsign failed

The `finish 3` was not being called when the `apk adbsign` command
failed within the `find` command using `-exec`. This happened because
`find` does not exit with a non-zero status when the command executed by
`-exec` fails, so the `|| finish 3` condition was not triggered.

So lets replace the `find ... -exec ...` construct with a loop and call
`finish 3` immediately if it fails.

Fixes: a94d4e15fdc1 ("add APK signing logic")
Signed-off-by: Petr Štetiar <ynezz@true.cz>