Don't set BUNDLER_ORIG environment variables

[peterzhu2118]

What was the end-user or developer problem that led to this PR?

Bundler creates a backup of environment variables by creating new environment variables prefixed with BUNDLER_ORIG_. This caused Ruby test suite's environment variable leak checker to report lots of created/changed environment variables (suppressed in commit ruby/ruby@6d83590).

What is your fix for the problem, implemented in this PR?

This PR stops creating BUNDLER_ORIG_ environment variables and instead relies on storing backups inside Ruby at boot.

However, this implementation causes a breaking change when spawning child processes. For example:

require "bundler"

ENV["BUNDLE_BIN_PATH"] = "foobar"

script = <<~RUBY
  require "bundler"

  Bundler.with_original_env do
    p ENV["BUNDLE_BIN_PATH"]
  end
RUBY

puts `bundle exec ruby -e '#{script}'`

Before this change it outputs nil while after this change it outputs "foobar".

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
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[hsbt]

👋 Dropping the BUNDLER_ORIG_* vars breaks the cross-process semantics of with_original_env/with_unbundled_env (your own example shows it).

Since this is really about quieting the test helper's env-leak checker, and ruby/ruby has lived fine with the suppression for ~4 years, the compatibility cost feels too steep.

Closing it, but thanks for your investigation.