CLI: individual file download does not work

[wonder-sk]

If I have a project and want to download a specific version, it's not working. Steps to reproduce:

1. download the project to a local directory

mergin --username john download john/myproject

2. go to the project's directory

cd myproject

3. download a file

mergin --username john download-file --version v273 myfile.gpkg /tmp/myfile-v273.gpkg

It would fail with an unhandled exception:

Unhandled exception!
Traceback (most recent call last):

  File "/home/martin/.local/lib/python3.8/site-packages/mergin/cli.py", line 338, in download_file
    job = download_file_async(mc, project_path, filepath, output, version)

  File "/home/martin/.local/lib/python3.8/site-packages/mergin/client_pull.py", line 604, in download_file_async
    mp = MerginProject(project_dir)

  File "/home/martin/.local/lib/python3.8/site-packages/mergin/merginproject.py", line 46, in __init__
    raise InvalidProject('Project directory does not exist')

mergin.common.InvalidProject: Project directory does not exist