Coder Connect hangs if you sign out and sign into a different deployment

[spikecurtis]

Reproduction steps:

1. Sign into a deployment
2. Turn on Coder Connect and wait for it to start
3. Turn off Coder Connect
4. Sign out
5. Sign into a different deployment (different URL)
6. Turn on Coder Connect

Coder Connect spinner will spin indefinitely and never come up.

We've traced it to Downloader.cs which maintains a ConcurrentDictionary called _downloads, which tracks in-progress download tasks keyed off destination path. We don't ever clear tasks out of this dictionary when they complete, so a new download, for a different URL gets stuck in a loop looking up the task in the dictionary, waiting for it (which returns instantly because it's done), and then noticing it's for a different URL, and retrying in a loop.