Use bundler-cache to bundle install and cache gems

[joshcooper]

The setup-ruby action makes it trivial to bundle install gems from a cache and update the cache when the run completes.

When using bundler-cache, we can't use bundle config commands, but we can achieve the same effect specifying BUNDLE_SET as an env variable.

See https://github.com/ruby/setup-ruby for details

On Ubuntu 22.04:

> bundle install
/opt/hostedtoolcache/Ruby/3.2.4/x64/bin/bundle config --local path /home/runner/work/puppet/puppet/vendor/bundle
/opt/hostedtoolcache/Ruby/3.2.4/x64/bin/bundle lock
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Writing lockfile to /home/runner/work/puppet/puppet/Gemfile.lock
Cache key: setup-ruby-bundler-cache-v6-ubuntu-22.04-x64-ruby-3.2.4-wd-/home/runner/work/puppet/puppet-with--without--only--Gemfile.lock-6185088470ded95016515539e14e1cd5f8944fabbb2851ece3a74dcc8e02f5f9
Cache Size: ~[24](https://github.com/puppetlabs/puppet/actions/runs/9574586858/job/26398059207?pr=9400#step:3:30) MB (24880861 B)
/usr/bin/tar -xf /home/runner/work/_temp/77788843-aafe-4128-b8ba-72781e8e961e/cache.tzst -P -C /home/runner/work/puppet/puppet --use-compress-program unzstd
Cache restored successfully
Found cache for key: setup-ruby-bundler-cache-v6-ubuntu-22.04-x64-ruby-3.2.4-wd-/home/runner/work/puppet/puppet-with--without--only--Gemfile.lock-6185088470ded95016515539e14e1cd5f8944fabbb[28](https://github.com/puppetlabs/puppet/actions/runs/9574586858/job/26398059207?pr=9400#step:3:34)51ece3a74dcc8e02f5f9
/opt/hostedtoolcache/Ruby/3.2.4/x64/bin/bundle install --jobs 4
Bundle complete! 34 Gemfile dependencies, 94 gems now installed.
Bundled gems are installed into `./vendor/bundle`
Took   3.36 seconds

Previously the "install ruby 3.2" and "update rubygems and install gems" steps took 1+17=18 seconds. Now the combined "install ruby 3.2" step took 4 seconds.