[Packaging] az upgrade: Detect correct cask token for preview vs production

During the preview phase (Plan B), the cask is distributed as
`azure-cli-preview` via a custom tap, while post-migration it
becomes `azure-cli` in homebrew-cask. The `az upgrade` command
now checks which cask token is installed and upgrades accordingly.

Author: naga-nandyala

src/azure-cli/azure/cli/command_modules/util/custom.py

@@ -132,9 +132,17 @@ def upgrade_version(cmd, update_all=None, yes=None, allow_preview=None):  # pyli
                 exit_code = subprocess.call(update_cmd)
         elif installer == 'HOMEBREW_CASK':
             logger.debug("Update homebrew cask")
+            # Determine cask token: 'azure-cli-preview' (custom tap) or 'azure-cli' (homebrew-cask)
+            cask_token = 'azure-cli'
+            try:
+                if subprocess.call(['brew', 'list', '--cask', 'azure-cli-preview'],
+                                   stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL) == 0:
+                    cask_token = 'azure-cli-preview'
+            except Exception:  # pylint: disable=broad-except
+                pass
             exit_code = subprocess.call(['brew', 'update'])
             if exit_code == 0:
-                update_cmd = ['brew', 'upgrade', '--cask', 'azure-cli']
+                update_cmd = ['brew', 'upgrade', '--cask', cask_token]
                 logger.debug("Update azure cli with '%s'", " ".join(update_cmd))
                 exit_code = subprocess.call(update_cmd)
         elif installer == 'PIP':