Zuul: Change nova-api-proxy-tox-pep8 nodeset

This Zuul job recently started failing to pull the requirements
it needs to run its tests.

It didn't have any nodeset defined and the default has recently
been changed from Ubuntu 22 [1] to Ubuntu 24 [2].

Setting its nodeset to debian-bullseye, matching the other tests
and the current STX base OS, fixed the issue.

Refs:
[1] Successfully pulled requirements on Ubuntu 22 (Jammy) - Aug 20
https://zuul.opendev.org/t/openstack/build/33b4d14e17784ca9ba6f098acbd9b1fc

[2] Failed to pull requirements on Ubuntu 24 (Noble) - Aug 24
https://zuul.opendev.org/t/openstack/build/d647556ebe074f29b58df5ba3d6ab5aa

Closes-Bug: 2077640

Change-Id: I335f25c5eaaa5b7d46ea15e47a7c68e7869bf8e4
Signed-off-by: Leonardo Fagundes Luz Serrano <Leonardo.FagundesLuzSerrano@windriver.com>

.zuul.yaml

@@ -76,6 +76,7 @@
     name: nova-api-proxy-tox-pep8
     parent: tox
     description: Run pep8 for nova-api-proxy
+    nodeset: debian-bullseye
     vars:
       tox_envlist: pep8
       tox_extra_args: -c nova-api-proxy/tox.ini