cask/installer: do not install tap during conflict checking

If the tap of the conflicted cask is not installed, we should just skip it.
This matches the behavior of formula installation.

Without this, running `brew install dropbox` would result to install
the `homebrew/cask-versions` tap.

Fixes #9125.

Library/Homebrew/cask/installer.rb

@@ -120,6 +120,11 @@ module Cask
       return unless @cask.conflicts_with
 
       @cask.conflicts_with[:cask].each do |conflicting_cask|
+        if (match = conflicting_cask.match(HOMEBREW_TAP_CASK_REGEX))
+          conflicting_cask_tap = Tap.fetch(match[1], match[2])
+          next unless conflicting_cask_tap.installed?
+        end
+
         conflicting_cask = CaskLoader.load(conflicting_cask)
         raise CaskConflictError.new(@cask, conflicting_cask) if conflicting_cask.installed?
       rescue CaskUnavailableError