cask: fewer GitHub Actions warnings.

- don't care about no checksums being defined for official casks
- don't complain about Gatekeeper being disabled on GitHub Actions as
  it's been globally disabled for the team

Library/Homebrew/cask/download.rb

@@ -82,7 +82,8 @@ def basename
 
     sig { override.params(filename: Pathname).void }
     def verify_download_integrity(filename)
-      if @cask.sha256 == :no_check
+      official_cask_tap = cask.tap&.official?
+      if @cask.sha256 == :no_check && !official_cask_tap
         opoo "No checksum defined for cask '#{@cask}', skipping verification."
         return
       end

Library/Homebrew/cask/installer.rb

@@ -107,7 +107,8 @@ def install
       backup if force? && @cask.staged_path.exist? && @cask.metadata_versioned_path.exist?
 
       oh1 "Installing Cask #{Formatter.identifier(@cask)}"
-      opoo "macOS's Gatekeeper has been disabled for this Cask" unless quarantine?
+      # GitHub Actions globally disables Gatekeeper.
+      opoo "macOS's Gatekeeper has been disabled for this Cask" if !quarantine? && !GitHub::Actions.env_set?
       stage
 
       @cask.config = @cask.default_config.merge(old_config)