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.