Since c1e7268c we install the latest
version of RuboCop in our GitHub Actions workflow for speed, but this
sacrifices reproducibility; the results will change whenever RuboCop
publishes a new version. Instead we can add a new group to our Gemfile
that just contains the dependencies necessary to run RuboCop, and skip
installing everything else in CI.

Unfortunately it's not possible to tell Bundler to only install gems
from a single group, so we have to tell it not to install every other
group instead.