Install gems for the Rails main target

bundler-cache: false makes ruby/setup-ruby skip bundle install entirely, so the
Rails main job reached bundle exec with no gems installed and failed with
"The git source https://github.com/rails/rails.git is not yet checked out".

Restore bundler-cache for every target and refresh the git source in a separate
step instead, which keeps the cache while still resolving the current branch
head.