Keith: Add Rust toolchain

modules/hosts/keith/devtools.nix

@@ -0,0 +1,9 @@
+{ inputs, ... }: {
+  flake.modules.nixos.keith-devtools = {
+    imports = with inputs.self.modules.nixos;
+      with inputs.self.factory; [
+        development-tools
+        rust-dev
+      ];
+  };
+}