arch-nspawn: Pass --as-pid2

As not all commands we run are capable of reaping processes correctly.
For example, pacman is not.

arch-nspawn.in

@@ -114,6 +114,6 @@ eval "$(grep '^CARCH=' "$working_dir/etc/makepkg.conf")"
 exec ${CARCH:+setarch "$CARCH"} systemd-nspawn -q \
 	-D "$working_dir" \
 	-E "PATH=/usr/local/sbin:/usr/local/bin:/usr/bin" \
-	--register=no --keep-unit \
+	--register=no --keep-unit --as-pid2 \
 	"${mount_args[@]}" \
 	"$@"