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.

flake.nix

@@ -12,10 +12,14 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    # Newer nixpkgs for yarr: the pinned nixpkgs has rustc 1.95 but yarr
+    # needs 1.97.1. Bumping the main nixpkgs input breaks aioimaplib
+    # (email server dep), so use a separate input just for yarr's rust.
+    rust-nixpkgs.url = "github:NixOS/nixpkgs/0e251e24a4f24e036a084b6b4b2d2491af4167f4";
   };
 
   outputs =
-    { self, nixpkgs, ... }:
+    { self, nixpkgs, rust-nixpkgs, ... }:
     let
       supportedSystems = [
         "x86_64-linux"
@@ -36,7 +40,10 @@
             inherit system;
             overlays = [ self.overlays.default ];
           };
-          mcpPackages = import ./packages { inherit pkgs; };
+          # yarr needs rustc 1.97.1; the main nixpkgs input only has 1.95.
+          # Build its rust toolchain from the newer rust-nixpkgs input.
+          rustPkgs = import rust-nixpkgs { inherit system; };
+          mcpPackages = import ./packages { inherit pkgs rustPkgs; };
         in
         mcpPackages // {
           default = pkgs.symlinkJoin {
@@ -53,7 +60,8 @@
             inherit system;
             overlays = [ self.overlays.default ];
           };
-          mcpPackages = import ./packages { inherit pkgs; };
+          rustPkgs = import rust-nixpkgs { inherit system; };
+          mcpPackages = import ./packages { inherit pkgs rustPkgs; };
         in
         {
           default = pkgs.mkShell {