Serve preview builds from a unique URL

Right now, we use the same download URL for all preview builds:

`https://github.com/coder/coder-desktop-macos/releases/download/preview/Coder-Desktop.pkg`

This means if an update popup advertises a preview build other than the latest (separately, why is this happening?) installing that build will fail, as the signature of the latest build does not match what Sparkle expected to download. This produces an update error:


This can be worked around by **simply checking for updates again**, and installing the advertised version, which will be the actual latest preview build.

This **does not effect `Stable` release builds**, as they are served from a unique URL:

`https://github.com/coder/coder-desktop-macos/releases/download/v0.7.0/Coder-Desktop.pkg`

We could solve this by serving the last N preview builds from a GCP bucket.