fix(cluster): vargroup "latest" doesn't work

[fubuloubu]

What I did

$ silverback cluster bots new -g <some vargroup> ...
...                                                                                                                              
  File "site-packages/silverback/_cli.py", line 859, in new_bot                                                      
    environment = [cluster.variable_groups[vg_name].get_revision("latest") for vg_name in vargroups]                                                                              
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                        
  File "site-packages/silverback/cluster/client.py", line 118, in get_revision                                       
    handle_error_with_response(response)                                                                                                                                          
  File "site-packages/silverback/cluster/client.py", line 54, in handle_error_with_response                          
    raise RuntimeError(message)                                                                                                                                                   
RuntimeError: Not Found

How I did it

How to verify it

I tested locally w/ staging cluster

Checklist

• Passes all linting checks (pre-commit and CI jobs)
  - [ ] New test cases have been added and are passing
  - [ ] Documentation has been updated
• PR title follows Conventional Commit standard (will be automatically included in the changelog)