Make typechecking work without setting gemfile

[Earlopain]

It's annoying having to set BUNDLE_GEMFILE manually for this

I tried simply putting it in the main gemfile but that would mean it's not usable with ruby-dev because of nokogiri.

Bundler tells you the right command to install gems if you haven't done that yet:

$ bundle exec rake typecheck:steep
bundle exec steep check
Could not find sexp_processor-4.17.4, unicode-emoji-4.1.0 in locally installed gems
Run `bundle install --gemfile gemfiles/typecheck/Gemfile` to install missing gems.
rake aborted!
Command failed with status (7): [bundle exec steep check]
/home/earlopain/Documents/ruby-prism/rakelib/typecheck.rake:82:in 'block (3 levels) in <top (required)>'
/home/earlopain/Documents/ruby-prism/rakelib/typecheck.rake:7:in 'block in Object#with_gemfile'
/home/earlopain/Documents/ruby-prism/rakelib/typecheck.rake:5:in 'Object#with_gemfile'
/home/earlopain/Documents/ruby-prism/rakelib/typecheck.rake:81:in 'block (2 levels) in <top (required)>'
/home/earlopain/.rbenv/versions/4.0.0/bin/bundle:25:in '<main>'
Tasks: TOP => typecheck:steep
(See full trace by running task with --trace)