Fix that bundler isn't installing in CI for Ruby 2.7

[pbyrne]

No description provided.

[pbyrne]

I think this is what did it, based on the earlier error message trying to use bundler: none:

The dependency memo_pad (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for arm64-darwin-23 but the dependency is only for ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, ruby, . To add those platforms to the bundle, run `bundle lock --add-platform ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby ruby `.
Unable to find a spec satisfying memo_pad (>= 0) in the set. Perhaps the
lockfile is corrupted?

I'm not sure what set that, maybe bundler init? But we want to install on any Ruby deployment, not just Mac on ARM.