Support pulling prebuilt ruby

[thii]

This currently supports building ruby from source and using ruby from host. Building ruby from source can be slow and has to be redone every time we upgrade our clang compiler. ruby coming from the host machine is outdated and is often incompatible with newer gems; it also creates a room for non-hermetic builds. It would be nice if this can have support for pulling prebuilt ruby binaries based on the host platform.

[kigster]

Looking for additional core maintainers: #146