Prevent random crash when autoloading Pathname

[deivid-rodriguez]

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

Not sure why, but I run into the following flaky test failure while working locally on something else

(...)

Invoking `/Users/deivid/.asdf/installs/ruby/3.1.2/bin/ruby -I/Users/deivid/Code/rubygems/rubygems/bundler/spec -r/Users/deivid/Code/rubygems/rubygems/bundler/spec/support/artifice/fail.rb -r/Users/deivid/Code/rubygems/rubygems/bundler/spec/support/hax.rb /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/bin/bundle install` failed with output:
----------------------------------------------------------------------
--- ERROR REPORT TEMPLATE -------------------------------------------------------

```
NameError: constant Pathname::FileUtils not defined

    FileUtils.mkpath(@path, mode: mode)
    ^^^^^^^^^
  /Users/deivid/.asdf/installs/ruby/3.1.2/lib/ruby/3.1.0/pathname.rb:585:in `mkpath'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/shared_helpers.rb:103:in `filesystem_access'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/rubygems_gem_installer.rb:78:in `build_extensions'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/rubygems_gem_installer.rb:28:in `install'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/source/rubygems.rb:207:in `install'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/installer/gem_installer.rb:54:in `install'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/installer/gem_installer.rb:16:in `install_from_spec'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/installer/parallel_installer.rb:186:in `do_install'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/installer/parallel_installer.rb:177:in `block in worker_pool'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/worker.rb:62:in `apply_func'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/worker.rb:57:in `block in process_queue'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/worker.rb:54:in `loop'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/worker.rb:54:in `process_queue'
  /Users/deivid/Code/rubygems/rubygems/bundler/tmp/4/gems/system/gems/bundler-2.4.0.dev/lib/bundler/worker.rb:91:in `block (2 levels) in create_threads'

(...)
```

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

Don't use Pathname at all here.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags