Ensure to use repo's Gemfile in application

Puma gets bundler's info from `Bundler::ORIGINAL_ENV` for restart.
https://github.com/puma/puma/blob/f6f3892f4d82638fb7a2a57d993641b1486ee88a/lib/puma/launcher.rb#L168

So, specified `BUNDLE_GEMFILE` env for use same Gemfile in the restart.

Fixes #31351

railties/test/application/server_test.rb

@@ -35,6 +35,11 @@ def teardown
     test "restart rails server with custom pid file path" do
       skip "PTY unavailable" unless available_pty?
 
+      File.open("#{app_path}/config/boot.rb", "w") do |f|
+        f.puts "ENV['BUNDLE_GEMFILE'] = '#{Bundler.default_gemfile.to_s}'"
+        f.puts "require 'bundler/setup'"
+      end
+
       master, slave = PTY.open
       pid = nil