ruby: add rubygems customization, fix postinstall

This configures rubygems to install all gems into a public, non-versioned directory by default - both gems and their binaries. (This means that users no longer need /usr/local/opt/ruby/bin in their PATH.)

This means that we no longer have to monkey with gem directories at all in postinstall. Bundled gems will be symlinked into this location.

Fixes #29982.
Closes #29986.
I considered leaving `private_dir` out, but this ensures it'll still work if ruby is unlinked.