Use bundler-cache to speed up CI

[joshcooper]

This updates the ruby based workflows to save/restore the bundler cache. See puppetlabs/puppet@86820ee for details.

This intentionally doesn't use bundler-cache for the mend workflow, since we
want the latest versions of all of our dependencies that still satisfy our
constraints.

This doesn't update the acceptance_tests workflow because it does a bundle
install for both the top-level Gemfile and the one in the acceptance dir.

All of the workflows are more than twice as fast, except for JRuby:

	Before	After
rubocop	1m 16s	41s
integration-ubuntu-ruby-2.7	1m 28s	36s
integration-ubuntu-ruby-3.2	56s	27s
integration-ubuntu-jruby	3m 19s	3m 17s
integration-windows-ruby-2.7	6m 15s	3m 53s
integration-windows-ruby-3.2	4m 51s	2m 45s
unit-ruby-2.5	1m 13s	22s
unit-ruby-2.7	1m 10s	23s
unit-ruby-3.0	38s	18s
unit-ruby-3.2	39s	13s
unit-jruby-9.3	1m 20s	1m 13s
unit-jruby-9.4	1m 27s	1m 19s
unit-windows-2.7	5m 18s	1m 58s
unit-windows-3.2	3m 52s	2m 37s

Coverage didn't run because it only triggers on push to main