Manually caching gems has many issues:
* Not working if there is no Gemfile.lock but only a Gemfile
* Not having the OS version in the key, which might cause binary incompatibility with system libraries of different ABI version.
* Not taking the Ruby version in account.
* Not taking the Ruby ABI version of development builds into accounts, which cannot be done with a key, but needs the commit hash.
* Using restore-keys would grow the cache over time and have extra gems in the cache.
* Those reasons are summarized in https://github.com/marketplace/actions/setup-ruby-jruby-and-truffleruby#caching-bundle-install-manually
