fix: detecting release version hangs for a long time (downloading assets)

[eseidel]

This has confused many customers. We don't explain that we're downloading Flutter assets.

One fix might be to just run flutter precache in its own separate step.

See
https://discord.com/channels/1030243211995791380/1229355551868321874
as on example.

[bryanoltman]

FWIW, "Detecting release version" is slow when Shorebird's cache (not Flutter's) is downloading artifacts (specifically here).

This particular issue wouldn't be solved by flutter precache, although precaching Flutter artifacts when a new version of Flutter is installed is a good idea – the first call to flutter build after installing a new version of Flutter takes a long time because assets are silently being downloaded).

[bryanoltman]

Looking into this. We just need to be careful to not cross logger.progress streams