Warn when Tapioca is executed without Bundler

[KaanOzkan]

Motivation

Addresses the finding in #1934 (comment)

Implementation

Print a warning as early as possible. Looking at various env variables BUNDLER_VERSION felt like a safe bet.

Tests

Tested manually. I can add it if wanted.

[graphite-app]

ENV['BUNDLER_VERSION'] is not a reliable indicator of Bundler being active, as it may be unset even when running under Bundler. Two more reliable alternatives are:

1. Check ENV['BUNDLE_GEMFILE'], which Bundler consistently sets
2. Check defined?(Bundler) to detect if Bundler is loaded

Either approach would prevent false warnings in valid Bundler environments.

Spotted by Graphite Reviewer

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[KaanOzkan]

I'm pretty sure BUNDLER_VERSION will be set https://github.com/rubygems/rubygems/blob/c46bf736318a41354c9e411c127247c764dff4ab/bundler/lib/bundler/shared_helpers.rb#L305

But we can do defined?(Bundler) too.

[egiurleo]

Is it possible to create a CLI test for this?

[KaanOzkan]

Is it possible to create a CLI test for this?

I tried shelling to tapioca without bundle exec in the test but Bundler is still available so I'm not sure how to test this.