when bundler is loaded, it sets the `RUBYOPT` environment variable to setup bundler. However, it was causing weird errors like the following when we try to install
custom plugin gems into a specific directory.
```
/home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/source/git.rb:214:in `rescue in load_spec_files': https://github.com/discourse/mail.git is not yet checked out. Run `bundle install` first. (Bundler::GitError)
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/source/git.rb:210:in `load_spec_files'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/source/path.rb:107:in `local_specs'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/source/git.rb:178:in `specs'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/lazy_specification.rb:88:in `__materialize__'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/spec_set.rb:75:in `block in materialize'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/spec_set.rb:72:in `map!'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/spec_set.rb:72:in `materialize'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/definition.rb:468:in `materialize'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/definition.rb:190:in `specs'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/definition.rb:238:in `specs_for'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/runtime.rb:18:in `setup'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler.rb:151:in `setup'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/setup.rb:20:in `block in <top (required)>'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/ui/shell.rb:136:in `with_level'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/ui/shell.rb:88:in `silence'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/bundler-2.3.5/lib/bundler/setup.rb:20:in `<top (required)>'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/site_ruby/2.7.0/rubygems/core_ext/kernel_require.rb:85:in `require'
from /home/tgxworld/.asdf/installs/ruby/2.7.5/lib/ruby/site_ruby/2.7.0/rubygems/core_ext/kernel_require.rb:85:in `require'
```
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