Simplestream generation fails for beta images

[Łukasz Zemczak]

When publishing beta images, I think this is something that got missed. Some tweaks to the code needs to be done. For now it's recommended to run publish scripts with `export SIMPLESTREAMS=0`.

Refreshing simplestreams...
Traceback (most recent call last):
  File "/home/cdimage/bin/publish-release", line 62, in <module>
    main()
  File "/home/cdimage/bin/publish-release", line 58, in main
    publisher.publish_release(source, date, publish_type)
  File "/srv/cdimage.ubuntu.com/bin/../lib/cdimage/tree.py", line 3554, in publish_release
    self.refresh_simplestreams()
  File "/srv/cdimage.ubuntu.com/bin/../lib/cdimage/tree.py", line 1732, in refresh_simplestreams
    sstreams.generate()
  File "/srv/cdimage.ubuntu.com/bin/../lib/cdimage/simplestreams.py", line 228, in generate
    self.scan_tree()
  File "/srv/cdimage.ubuntu.com/bin/../lib/cdimage/simplestreams.py", line 341, in scan_tree
    self.scan_releases_project(project_dir, entry)
  File "/srv/cdimage.ubuntu.com/bin/../lib/cdimage/simplestreams.py", line 326, in scan_releases_project
    self.scan_target(target_dir, series, project, None, None)
  File "/srv/cdimage.ubuntu.com/bin/../lib/cdimage/simplestreams.py", line 182, in scan_target
    for file in os.listdir(target_dir):
FileNotFoundError: [Errno 2] No such file or directory: '/srv/cdimage.ubuntu.com/www/full/xubuntu/releases/jammy/release'