Bundler re-exec breaks programs read from standard input

This is closely related to #8106.

Since commit https://github.com/rubygems/rubygems/commit/076aba8b1c25291a986f9c233cb80192adbe2fcf it's not possible to reliably feed a program to ruby on standard input.

If:

• Gemfile.lock was created by a version of bundler which is not in the global gem list; AND
• bundled gems are installed in a vendored directory; AND
• program is read from standard input,

Then:

• the program does not execute.

Files

bug.rb

require 'bundler/setup'
puts Bundler::VERSION

Gemfile

source 'https://rubygems.org'

Gemfile.lock (note: assuming 2.5.22 is not in the global gemset)

GEM
  remote: https://rubygems.org/
  specs:

PLATFORMS
  ruby
  x86_64-linux

DEPENDENCIES

BUNDLED WITH
   2.5.22

Steps to reproduce

$ bundle config set deployment true
$ bundle install
Bundler 2.5.23 is running, but your lockfile was generated with 2.5.22. Installing Bundler 2.5.22 and restarting using that version.
Fetching gem metadata from https://rubygems.org/.
Fetching bundler 2.5.22
Installing bundler 2.5.22
The Gemfile specifies no dependencies
Bundle complete! 0 Gemfile dependencies, 1 gem now installed.
Bundled gems are installed into `./vendor/bundle`

$ ruby bug.rb
2.5.22
$ ruby < bug.rb
$

Reproduce the failure with bug.rb, Gemfile, and Gemfile.lock using the deployment install steps, then compare stdin execution with ruby bug.rb. Start by reading the Bundler re-exec behavior introduced in commit 076aba8b1c25291a986f9c233cb80192adbe2fcf. Done means ruby < bug.rb executes and prints Bundler::VERSION under the specified lockfile and vendor conditions.