Add missing fileutils require in rubygems installer

[deivid-rodriguez]

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

When bundling a git gem including executables, the second time bundle install
is run (when the gem has been cached), we would get a crash like:

NameError: uninitialized constant Gem::Installer::FileUtils
Did you mean?  FileTest
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/rubygems/installer.rb:536:in `generate_bin_script'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/rubygems/installer.rb:506:in `block in generate_bin'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/rubygems/installer.rb:482:in `each'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/rubygems/installer.rb:482:in `generate_bin'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/source/path/installer.rb:46:in `generate_bin'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/source/path/installer.rb:36:in `post_install'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/source/path.rb:242:in `generate_bin'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/source/git.rb:184:in `install'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer/gem_installer.rb:67:in `install'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer/gem_installer.rb:18:in `install_from_spec'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer/parallel_installer.rb:163:in `do_install'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer/parallel_installer.rb:148:in `install_serially'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer/parallel_installer.rb:99:in `call'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer/parallel_installer.rb:78:in `call'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer.rb:285:in `install_in_parallel'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer.rb:209:in `install'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer.rb:91:in `block in run'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/process_lock.rb:12:in `block in lock'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/process_lock.rb:9:in `open'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/process_lock.rb:9:in `lock'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer.rb:72:in `run'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/installer.rb:24:in `install'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli/install.rb:64:in `run'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:262:in `block in install'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/settings.rb:116:in `temporary'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:261:in `install'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:30:in `dispatch'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/cli.rb:24:in `start'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.0.rc.2/exe/bundle:49:in `block in <top (required)>'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/site_ruby/2.7.0/bundler/friendly_errors.rb:117:in `with_friendly_errors'
  /home/deivid/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.0.rc.2/exe/bundle:37:in `<top (required)>'
  /home/deivid/.rbenv/versions/2.7.2/bin/bundle:23:in `load'
  /home/deivid/.rbenv/versions/2.7.2/bin/bundle:23:in `<main>'
  /home/deivid/.rbenv/versions/2.7.2/bin/ruby_executable_hooks:24:in `eval'
  /home/deivid/.rbenv/versions/2.7.2/bin/ruby_executable_hooks:24:in `<main>'

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

Bundler inherits from rubygems installer and uses this method directly. It can happen that fileutils is not yet required at that point.

To repro the issue I needed to make sure that the executables inside dummy specs have the executable bit set. Otherwise a different code path that does require fileutils is run. This seems like a good thing anyways since it better resembles the real world.

The fix is to add the missing require.

Make sure he 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