Replace Gemfile with Gemfile.next

[arielj]

This PR replaces all references of Gemfile and Gemfile.lock with Gemfile.next and Gemfile.next.lock.

[JuanVqz]

@arielj I noticed we need to set the BUNDLE_GEMFILE=Gemfile.next to make this work because we are still reading it, and if for some reason they forget to set it to the Gemfile.next, they might run into runtime issues as the previous approach.

This is due to buildpack reads Gemfile.next at build but dyno defaults to Gemfile/Gemfile.lock at runtime.

Here is the test I ran if you want to copy-paste:

1. confirm app uses the PR buildpack

heroku buildpacks -a app-staging

2. the manual config var that makes runtime work

heroku config -a app-staging | grep -i bundle

3. the two lockfiles differ

heroku run --no-tty -a app-staging 'grep "^    rails " Gemfile.lock; grep "^    rails " Gemfile.next.lock'

4. Probe A: current behavior (config var present): WORKS

heroku run --no-tty -a app-staging 'bundle exec ruby -e "require %q{rails}; puts Rails::VERSION::STRING"'

5. Probe B: simulate no-fix state (force default Gemfile): CRASHES

heroku run --no-tty -a app-staging 'BUNDLE_GEMFILE=$HOME/Gemfile bundle exec ruby -e "require %q{rails}; puts Rails::VERSION::STRING"'

Logs when running Probe B:

➜  heroku-buildpack-ruby-gemfile-next git:(hardcode-gemfile-next) ✗ heroku run --no-tty -a app-staging 'BUNDLE_GEMFILE=$HOME/Gemfile bundle exec ruby -e "require %q{rails}; puts Rails::VERSION::STRING"'                                   
Running BUNDLE_GEMFILE=$HOME/Gemfile bundle exec ruby -e "require %q{rails}; puts Rails::VERSION::STRING" on ⬢ app-staging... up, run.7494
/app/vendor/bundle/ruby/4.0.0/gems/bundler-4.0.10/lib/bundler/definition.rb:703:in 'Bundler::Definition#materialize': Could not find rails-8.0.3, sidekiq-7.3.9, railties-8.0.3, activesupport-8.0.3, thor-1.5.0, actionpack-8.0.3, activemodel-8.0.3, rack-3.2.6, actioncable-8.0.3, actionmailbox-8.0.3, actionmailer-8.0.3, actiontext-8.0.3, actionview-8.0.3, activejob-8.0.3, activerecord-8.0.3, activestorage-8.0.3, i18n-1.14.8, redis-client-0.28.0, bigdecimal-4.1.2, concurrent-ruby-1.3.6, logger-1.7.0, connection_pool-2.5.5, irb-1.18.0, rackup-2.3.1, rake-13.4.2, zeitwerk-2.7.5, nokogiri-1.19.3-x86_64-linux-gnu, benchmark-0.5.0, minitest-6.0.6, rack-session-2.1.2, rails-html-sanitizer-1.7.0, json-2.19.3, mail-2.8.1, rdoc-7.2.0, reline-0.6.3, loofah-2.25.1, net-imap-0.5.12, erb-6.0.4 in locally installed gems (Bundler::GemNotFound)
	from /app/vendor/bundle/ruby/4.0.0/gems/bundler-4.0.10/lib/bundler       /definition.rb:240:in 'Bundler::Definition#specs'
	from /app/vendor/bundle/ruby/4.0.0/gems/bundler-4.0.10/lib/bundler       /definition.rb:313:in 'Bundler::Definition#specs_for'
	from /app/vendor/bundle/ruby/4.0.0/gems/bundler-4.0.10/lib/bundler       /runtime.rb:18:in 'Bundler::Runtime#setup'
	from /app/vendor/bundle/ruby/4.0.0/gems/bundler-4.0.10/lib/bundler       .rb:165:in 'Bundler.setup'
	from /app/vendor/bundle/ruby/4.0.0/gems/bundler-4.0.10/lib/bundler       /setup.rb:32:in 'block in <top (required)>'
	from /app/vendor/bundle/ruby/4.0.0/gems/bundler-4.0.10/lib/bundler       /ui/shell.rb:177:in 'Bundler::UI::Shell#with_level'
	from /app/vendor/bundle/ruby/4.0.0/gems/bundler-4.0.10/lib/bundler       /ui/shell.rb:122:in 'Bundler::UI::Shell#silence'
	from /app/vendor/bundle/ruby/4.0.0/gems/bundler-4.0.10/lib/bundler       /setup.rb:32:in '<top (required)>'
	from /app/vendor/ruby-4.0.2/lib/ruby/4.0.0/rubygems.rb:1455:in 'Ke       rnel#require'
	from /app/vendor/ruby-4.0.2/lib/ruby/4.0.0/rubygems.rb:1455:in '<t       op (required)>'
	from <internal:gem_prelude>:2:in '<internal:gem_prelude>'

For that reason, I'm suggesting "forcing" the usage of the Gemfile.next and the user does not need to set the BUNDLE_GEMFILE anymore.

Thoughts?

[JuanVqz]

Force the running dyno to use Gemfile.next/Gemfile.next.lock instead of bundler's default Gemfile/Gemfile.lock

Suggested change

	set_env_default "BUNDLE_DEPLOYMENT", ENV["BUNDLE_DEPLOYMENT"] if ENV["BUNDLE_DEPLOYMENT"] # Unset on windows since we delete the Gemfile.next.lock
	set_env_default "BUNDLE_DEPLOYMENT", ENV["BUNDLE_DEPLOYMENT"] if ENV["BUNDLE_DEPLOYMENT"] # Unset on windows since we delete the Gemfile.next.lock
	set_env_default "BUNDLE_GEMFILE", "$HOME/Gemfile.next"