Source URL has been changed, things are going to break soon until I publish an update.
The new one is https://cdn.winget.microsoft.com/cache/source.msix.
I need to publish an update for it quickly so everything doesn't completely break. While I'm doing that, I'll add the long file path support into the manifest so it works for at least the people who have set up long file path support manually.
See here: https://github.com/microsoft/winget-cli/pull/2026/files
Ideally, editing sources would already be available, but I just haven't gotten around to it yet.
Update: the old URL at https://winget.azureedge.net/cache/source.msix still works for now.
Version 0.3.0.2 changes this to use the new URL.
This will be closed once it's changed in v0.4.
It's changed now, finally.