ci: switch to rustup to get rust 1.95+ for bevy_ecs 0.19
bevy_ecs 0.19 and edition 2024 require rustc >=1.95 which is newer than
the cargo package in nixery's nixpkgs snapshot. Switch all workflows from
the nixpkgs cargo/rustfmt packages to rustup installing stable at runtime.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>