cask: add new --cache command

It prints the file used to cache Casks.

This can be used to help users to install casks with their files
downloaded from outside brew.

See #6157.

Library/Homebrew/cask/cmd.rb

@@ -10,6 +10,7 @@ require "cask/config"
 require "cask/cmd/options"
 
 require "cask/cmd/abstract_command"
+require "cask/cmd/--cache"
 require "cask/cmd/audit"
 require "cask/cmd/automerge"
 require "cask/cmd/cat"

Library/Homebrew/cask/cmd/--cache.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+require "cask/download"
+
+module Cask
+  class Cmd
+    class Cache < AbstractCommand
+      def self.command_name
+        "--cache"
+      end
+
+      def initialize(*)
+        super
+        raise CaskUnspecifiedError if args.empty?
+      end
+
+      def run
+        casks.each do |cask|
+          puts Download.new(cask).downloader.cached_location
+        end
+      end
+
+      def self.help
+        "display the file used to cache the Cask"
+      end
+    end
+  end
+end