Currently we get the following warnings on `ruby setup.rb`:

```
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/exceptions.rb:281: warning: already initialized constant Gem::UnsatisfiableDepedencyError
/home/deivid/Code/rubygems/lib/rubygems/exceptions.rb:281: warning: previous definition of UnsatisfiableDepedencyError was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/user_interaction.rb:557: warning: already initialized constant Gem::StreamUI::ThreadedDownloadReporter::MUTEX
/home/deivid/Code/rubygems/lib/rubygems/user_interaction.rb:557: warning: previous definition of MUTEX was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/ext/builder.rb:20: warning: already initialized constant Gem::Ext::Builder::CHDIR_MUTEX
/home/deivid/Code/rubygems/lib/rubygems/ext/builder.rb:20: warning: previous definition of CHDIR_MUTEX was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/ext/ext_conf_builder.rb:14: warning: already initialized constant Gem::Ext::ExtConfBuilder::FileEntry
/home/deivid/Code/rubygems/lib/rubygems/ext/ext_conf_builder.rb:14: warning: previous definition of FileEntry was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/version.rb:158: warning: already initialized constant Gem::Version::VERSION_PATTERN
/home/deivid/Code/rubygems/lib/rubygems/version.rb:158: warning: previous definition of VERSION_PATTERN was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/version.rb:159: warning: already initialized constant Gem::Version::ANCHORED_VERSION_PATTERN
/home/deivid/Code/rubygems/lib/rubygems/version.rb:159: warning: previous definition of ANCHORED_VERSION_PATTERN was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/requirement.rb:14: warning: already initialized constant Gem::Requirement::OPS
/home/deivid/Code/rubygems/lib/rubygems/requirement.rb:14: warning: previous definition of OPS was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/requirement.rb:24: warning: already initialized constant Gem::Requirement::SOURCE_SET_REQUIREMENT
/home/deivid/Code/rubygems/lib/rubygems/requirement.rb:24: warning: previous definition of SOURCE_SET_REQUIREMENT was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/requirement.rb:27: warning: already initialized constant Gem::Requirement::PATTERN_RAW
/home/deivid/Code/rubygems/lib/rubygems/requirement.rb:27: warning: previous definition of PATTERN_RAW was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/requirement.rb:32: warning: already initialized constant Gem::Requirement::PATTERN
/home/deivid/Code/rubygems/lib/rubygems/requirement.rb:32: warning: previous definition of PATTERN was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/requirement.rb:37: warning: already initialized constant Gem::Requirement::DefaultRequirement
/home/deivid/Code/rubygems/lib/rubygems/requirement.rb:37: warning: previous definition of DefaultRequirement was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/requirement.rb:42: warning: already initialized constant Gem::Requirement::DefaultPrereleaseRequirement
/home/deivid/Code/rubygems/lib/rubygems/requirement.rb:42: warning: previous definition of DefaultPrereleaseRequirement was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/requirement.rb:311: warning: already initialized constant Gem::Version::Requirement
/home/deivid/Code/rubygems/lib/rubygems/requirement.rb:311: warning: previous definition of Requirement was here
/home/deivid/.rbenv/versions/2.7.1/lib/ruby/site_ruby/2.7.0/rubygems/command.rb:626: warning: already initialized constant Gem::Command::HELP
/home/deivid/Code/rubygems/lib/rubygems/command.rb:626: warning: previous definition of HELP was here
  Successfully built RubyGem
  Name: bundler
  Version: 2.2.0.dev
  File: bundler-2.2.0.dev.gem
Bundler 2.2.0.dev installed
RubyGems 3.2.0.pre1 installed
Regenerating binstubs
Regenerating plugins

------------------------------------------------------------------------------

RubyGems installed the following executables:
	/home/deivid/.rbenv/versions/2.7.1/bin/gem
	/home/deivid/.rbenv/versions/2.7.1/bin/bundle

```

This is because the `$LOAD_PATH` entry added by `setup.rb` is relatively
and when the offending require happens, we're installing `bundler` and
have switched folders to `bundler/`. So the require fallsback to the
system rubygems.

To avoid that, add an absolute path to the `$LOAD_PATH`.

On jruby, somehow the $LOAD_PATH is manipulated so that we end up
requiring stuff inside the built package even if we have specified the
`-I` flag, so we get redefinition warnings anyways.

I'm not sure about the root cause, but relative requiring fixes it, and
it's faster anyways.