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.

Inizia tracciando come vengono pubblicate le build di anteprima e come l’updater risolve l’URL di download dell’anteprima, usando l’URL stabile con versione come punto di confronto. Verifica l’approccio proposto del bucket GCP rispetto al flusso di release e aggiornamento; il lavoro è completo quando le build di anteprima annunciate in precedenza vengono scaricate correttamente e le relative firme corrispondono.

Scritto dal modello di indicizzazione a partire dal testo della issue.