bundler/inline: perform installation from a forked child

Unless of course fork isn't available.

Alternate: #7930, #7933
Fix: #7930, #7933

When bundler inline has to install gems, it loads more dependencies than when it
goes through the fast path of all gems being installed.

One of them is `securerandom` so if trying to use bundler/inline with a gem that
have a dependency on securerandom that don't match the default version, the script
fails with `Gem::LoadError`.

This can be preproduced on Ruby 3.2.x, after making sure to `gem uninstall securerandom`
so only the default gem remains, and then running the following script:

```ruby
require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'activesupport', '7.2.0' # depends on securerandom >= 0.3
end

require 'securerandom'
```

Author: byroot

bundler/lib/bundler/inline.rb

@@ -54,12 +54,26 @@ def definition.lock(*); end
       definition.validate_runtime!
 
       if install || definition.missing_specs?
-        Bundler.settings.temporary(inline: true, no_install: false) do
-          installer = Bundler::Installer.install(Bundler.root, definition, system: true)
-          installer.post_install_messages.each do |name, message|
-            Bundler.ui.info "Post-install message from #{name}:\n#{message}"
+        do_install = -> do
+          Bundler.settings.temporary(inline: true, no_install: false) do
+            installer = Bundler::Installer.install(Bundler.root, definition, system: true)
+            installer.post_install_messages.each do |name, message|
+              Bundler.ui.info "Post-install message from #{name}:\n#{message}"
+            end
           end
         end
+
+        # When possible we do the install in a subprocess because to install
+        # gems we need to require some default gems like `securerandom` etc
+        # which may later conflict with the Gemfile requirements.
+        if Process.respond_to?(:fork)
+          _, status = Process.waitpid2(Process.fork(&do_install))
+          exit(status.exitstatus || status.to_i) unless status.success?
+
+          return gemfile(false, options, &gemfile)
+        else
+          do_install.call
+        end
       end
 
       runtime = Bundler::Runtime.new(nil, definition)