refactor: delegate Rust toolchain management to rustup

Replace nixpkgs-provided cargo and rust-analyzer with rustup,
allowing rustup to manage the complete Rust toolchain outside
of Nix control.

Author: refnode

users/refnode/default.nix

@@ -120,8 +120,7 @@
     unstable.sqlite
     unstable.mob
     pkgs.garden
-    unstable.cargo
-    unstable.rust-analyzer
+    unstable.rustup
     unstable.talosctl
     unstable.cilium-cli
     unstable.jujutsu