jenkins nodes: limit fs.nr_open to something bookworm like, thanks elbrus

trixie's systemd raises the amount of allowed open fd's to 1e9. Some tests in
bookworm misbehave, e.g. websockify cf bug 1075849. As I don't want to play
whack-a-mole, lets ensure the we limit the amount of file descriptors until we
can do this per suite.

references:
https://lists.debian.org/debian-devel/2024/06/msg00041.html
https://bugs.debian.org/1075849
ci-team/debian-ci-config@632cc1e4



Signed-off-by: Holger Levsen <holger@layer-acht.org>