Fabrik can build Cargo-backed Rust binaries and dogfoods its own release builds

This release adds a cargo_binary target so you can build Cargo-backed Rust binaries with Fabrik. It also introduces a root //:fabrik target so the release binary can build Fabrik through Fabrik, and the README now documents the dogfood commands. The mise environment is now passed through to these dogfood builds so the correct toolchain is available.