Cache current working directory to speed up Rubocop

This saves thousands of getcwd() system calls and improves the
performance of running Rubocop, which is also used by HAML linting.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/61383

rubocop/qa_helpers.rb

@@ -5,7 +5,7 @@ module RuboCop
     def in_qa_file?(node)
       path = node.location.expression.source_buffer.name
 
-      path.start_with?(File.join(Dir.pwd, 'qa'))
+      path.start_with?(File.join(RuboCop::PathUtil.pwd, 'qa'))
     end
   end
 end

rubocop/spec_helpers.rb

@@ -6,14 +6,16 @@ module RuboCop
     # Returns true if the given node originated from the spec directory.
     def in_spec?(node)
       path = node.location.expression.source_buffer.name
+      pwd = RuboCop::PathUtil.pwd
 
       !SPEC_HELPERS.include?(File.basename(path)) &&
-        path.start_with?(File.join(Dir.pwd, 'spec'), File.join(Dir.pwd, 'ee', 'spec'))
+        path.start_with?(File.join(pwd, 'spec'), File.join(pwd, 'ee', 'spec'))
     end
 
     def migration_directories
       @migration_directories ||= MIGRATION_SPEC_DIRECTORIES.map do |dir|
-        [File.join(Dir.pwd, dir), File.join(Dir.pwd, 'ee', dir)]
+        pwd = RuboCop::PathUtil.pwd
+        [File.join(pwd, dir), File.join(pwd, 'ee', dir)]
       end.flatten
     end