build: replace deprecated method

    $ bundle exec rake build:all
    [DEPRECATED] `Bundler.with_clean_env` has been deprecated in
    favor of `Bundler.with_unbundled_env`. If you instead want
    the environment before bundler was originally loaded, use
    `Bundler.with_original_env`

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Rakefile

@@ -13,7 +13,7 @@ task test: [:base_test]
 namespace :build do
   desc 'Build gems for all platforms'
   task :all do
-    Bundler.with_clean_env do
+    Bundler.with_original_env do
       %w[ruby x86-mingw32 x64-mingw32 x64-mingw-ucrt].each do |name|
         ENV['GEM_BUILD_FAKE_PLATFORM'] = name
         Rake::Task["build"].execute