build(flake): add Rust toolchain (cargo/rustc/clippy/rustfmt) to the devShell

Groundwork for a native Rust printable-binary implementation (Rust GUI <-> Zig
core transport). The local rustup toolchain on this machine is broken (missing
libstd dylib), so the sandboxed nix toolchain is the path for both dev and CI.

Author: pmarreck

flake.nix

@@ -202,6 +202,12 @@
 
               # WebAssembly toolchain
               emscripten
+
+              # Rust toolchain (native Rust GUI<->core transport implementation)
+              cargo
+              rustc
+              clippy
+              rustfmt
             ]  ++ lib.optionals stdenv.isDarwin [ lldb ]
                ++ lib.optionals (!stdenv.isDarwin) [ gdb ]
                ++ linuxOnly;