yarr: add flake input for rustc 1.97.1, exclude from overlay

yarr needs rustc 1.97.1 but the main nixpkgs input (pinned by flake.lock)
has rustc 1.95.0. Bumping the main input breaks aioimaplib (email server
dep), so add a separate rust-nixpkgs input pinned to a newer rev.

yarr is excluded from the overlay (which uses the main nixpkgs) — build
it via 'nix build .#yarr' on the flake directly.

overlays/default.nix

@@ -16,8 +16,11 @@
 #     };
 #   }
 #
+# Note: yarr is excluded from the overlay because it needs rustc 1.97.1
+# from a specific nixpkgs input. Build it via nix build .#yarr on the
+# flake directly.
 final: prev:
 let
   mcpPackages = import ../packages { pkgs = final; };
 in
-mcpPackages
+builtins.removeAttrs mcpPackages [ "yarr" ]