update_environments workflow deduplicates common variants but not backends

With this workflow-template, udpate_environments generates two sets of images, not 3.

static_parameters:
  vendor: debian
  targets:
  - backends: ['unshare']
    codenames: ['sid', 'trixie']
    architectures: ['amd64']
    codename_aliases:
      sid: ['unstable', 'experimental']
    mmdebstrap_template:
      bootstrap_options:
        variant: minbase
      bootstrap_repositories:
      - mirror: http://deb.debian.org/debian
        components: ['main']
  - backends: ['unshare']
    variants: ['sbuild']
    codenames: ['sid', 'trixie']
    architectures: ['amd64']
    codename_aliases:
      sid: ['unstable', 'experimental']
    mmdebstrap_template:
      bootstrap_options:
        variant: buildd
      bootstrap_repositories:
      - mirror: http://deb.debian.org/debian
        components: ['main']
  - backends: ['incus-lxc']
    codenames: ['sid', 'trixe']
    architectures: ['amd64']
    codename_aliases:
      sid: ['unstable', 'experimental']
    mmdebstrap_template:
      bootstrap_options:
        variant: buildd
        extra_packages:
        - dbus
        - systemd
        - systemd-resolved
        - systemd-sysv
      bootstrap_repositories:
      - mirror: http://deb.debian.org/debian
        components: ['main']
runtime_parameters:
  task_configuration: {}

This is because the group for each work-request is generated from only (codename, variants) not (codename, variants, backends). Maybe group should just be target-$i ?

https://salsa.debian.org/freexian-team/debusine/-/blob/51b9140e41d6a4677e349c4be6eed0ab06b4bbd6/debusine/server/workflows/update_environments.py#L237-241