Use constants for RuboCop version.

Library/Homebrew/cask/lib/hbc/cli/style.rb

@@ -23,12 +23,10 @@ module Hbc
         $CHILD_STATUS.success?
       end
 
-      RUBOCOP_CASK_VERSION = "~> 0.10.6".freeze
-
       def install_rubocop
         Utils.capture_stderr do
           begin
-            Homebrew.install_gem_setup_path! "rubocop-cask", RUBOCOP_CASK_VERSION, "rubocop"
+            Homebrew.install_gem_setup_path! "rubocop-cask", HOMEBREW_RUBOCOP_CASK_VERSION, "rubocop"
           rescue SystemExit
             raise CaskError, Tty.strip_ansi($stderr.string).chomp.sub(/\AError: /, "")
           end

Library/Homebrew/cmd/style.rb

@@ -47,7 +47,7 @@ module Homebrew
 
   def check_style_impl(files, output_type, options = {})
     fix = options[:fix]
-    Homebrew.install_gem_setup_path! "rubocop", "0.47.1"
+    Homebrew.install_gem_setup_path! "rubocop", HOMEBREW_RUBOCOP_VERSION
 
     args = %w[
       --force-exclusion

Library/Homebrew/config.rb

@@ -48,3 +48,7 @@ end
 
 # Load path used by standalone scripts to access the Homebrew code base
 HOMEBREW_LOAD_PATH = HOMEBREW_LIBRARY_PATH
+
+# RuboCop version used for `brew style` and `brew cask style`
+HOMEBREW_RUBOCOP_VERSION = "0.47.1".freeze
+HOMEBREW_RUBOCOP_CASK_VERSION = "~> 0.10.6".freeze # has to be updated when RuboCop version changes