start: switch ids at last possible instance

start: switch ids at last possible instance

This is technically not necessary but it is a privilege sensitive operation.
Meaning if anyone wants to do something that requires privilege it should be
done before the id switch. So let's move the id switch immediately before the
exec so that it's called at the last possible moment.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>