Update signal-beta to use .dmg instead of .zip

Getting signing issues with the downloaded .zip

As you can see [here](signalapp/Signal-Desktop#3128) it appears that others have, before, experienced issues with the .zip version.

I'm not sure why there are two, but .dmg seems to not have any signing issues. Would appreciate thoughts from others as to why this issue is occurring with the .zip version.

Thanks!

Peter

Casks/signal-beta.rb

@@ -1,8 +1,8 @@
 cask "signal-beta" do
   version "1.39.6-beta.1"
-  sha256 "8782ded1308e21f21417096d76819f777279472502727555f3ae0836d2d4a1c1"
+  sha256 "7bfd120c1456d5f0b1b4d0188d682141e405b5c07bbf76398bf61a50987817d8"
 
-  url "https://updates.signal.org/desktop/signal-desktop-beta-mac-#{version}.zip"
+  url "https://updates.signal.org/desktop/signal-desktop-beta-mac-#{version}.dmg"
   appcast "https://github.com/signalapp/Signal-Desktop/releases.atom"
   name "Signal Beta"
   homepage "https://signal.org/"