Add support for compiling Rust

Rust is fast becoming a requirement for Ruby-related projects:

1. Rust port of prometheus-client-mmap: gitlab-org/ruby/gems/prometheus-client-mmap!79 (closed)
2. Other gems like commonmarker
3. Ruby 3.2's YJIT

We should ship rustc with gitlab-omnibus-builder. !260 (merged) appears to work.