This PR brings back https://github.com/zed-industries/zed/pull/30969 and
adds some initial testing.

https://github.com/zed-industries/zed/pull/30969 did indeed allow Zed to
continue doing downloads after downloading one, but it introduced a bug
where Zed would download a new binary every time it polled, even if the
version was the same as the running instance.

This code could use a refactor to allow more / better testing, but this
is a start.

Release Notes:

- N/A