[Feature Request] Clean up stale `.downloading` ephemeral server files

[bergundy]

Is your feature request related to a problem? Please describe.

When downloading an ephemeral server fails or is aborted, we may end up leaving a .downloading file in the user's temp directory preventing new downloads from starting when the user is retries running their tests.

Describe the solution you'd like

Clean up stale (based on mtime?) .downloading files and automatically retry downloads.