nixos/stage1: Fix local keyword outside function

Thanks to @NeQuissimus in a5c1985f for
updating busybox, which since version 1.25 doesn't allow local variables
outside of functions anymore (which is the desired behaviour).

See the following upstream commit of busybox which is the change that
let's this problem surface:

https://git.busybox.net/busybox/commit/?id=ef2386b80abfb22ccb697ddbdd4047aacc395c50

So this has been an error I've made on my end in
67223ee2

, because I originally had a
function for killing the processes but desired to inline it because it's
only used in one place.

This fixes the boot-stage1 NixOS test.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>