Make sure we respect bundler config for Gemfile

What does this MR do?

Make sure we respect bundler config for Gemfile.

When we run bundle exec, it'll go through: https://github.com/rubygems/rubygems/blob/v3.2.26/bundler/lib/bundler/cli.rb#L58-L60 namely:

custom_gemfile = options[:gemfile] || Bundler.settings[:gemfile]
if custom_gemfile && !custom_gemfile.empty?
  Bundler::SharedHelpers.set_env "BUNDLE_GEMFILE", File.expand_path(custom_gemfile)
  Bundler.reset_settings_and_root!
end

So it'll respect the config for Gemfile in the bundle config. However, if we just require 'bundler/setup' then the above code won't run and the config will be ignored.

This attempts to bring them aligned.

This change is tested at gitlab-jh/gitlab!89 (closed)

• compile-test-assets before: https://gitlab.com/gitlab-jh/gitlab/-/jobs/1556019024
• compile-test-assets after: https://gitlab.com/gitlab-jh/gitlab/-/jobs/1556953791
• setup-test-env before: https://gitlab.com/gitlab-jh/gitlab/-/jobs/1556019029
• setup-test-env after: https://gitlab.com/gitlab-jh/gitlab/-/jobs/1556953797

Considerations:

• Document this
• Add a Rubocop cop to detect require 'setup/bundler' because it'll not work with #338469 (closed)

Related to #339939 (closed)