Update horizon cross-npm job to define a node_version

As of Id2e26a6099ba276feb804ce49a6b591282cc2b0c the default node_version has
been removed.  Incidentally the default was 14 which is just plain wrong.

So lets add a value into our cross job vars.  Use something like:
    cd path/to/horizon
    git grep node_version origin/stable/2024.1 -- .zuul.d
to pick a likely value.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Change-Id: Ib3358c3cae9fc196ba9f2aa41a6c50be4eb4be47

Author: tbreeds

.zuul.d/cross-jobs.yaml

@@ -40,6 +40,7 @@
     vars:
       zuul_work_dir: "{{ (zuul.projects.values() | selectattr('required') | selectattr('name', 'match', '^(?!openstack/requirements)') | list)[0].src_dir }}"
       tox_constraints_file: "{{ ansible_user_dir }}/{{ zuul.projects['opendev.org/openstack/requirements'].src_dir }}/upper-constraints.txt"
+      node_version: 20
     pre-run: playbooks/nodejs-pre.yaml
     files:
       - upper-constraints.txt