yarr: use pre-built binary instead of building from source

Building yarr from source (Rust) takes ~5+ min and times out the
config-nixos CI runner (3h limit) when combined with the full
perlis+wilkes+relay flake check. Use the pre-built x86_64-linux binary
from GitHub releases instead — fast fetch, no Rust toolchain needed.

Removes the rust-nixpkgs flake input (no longer needed).

flake.nix

@@ -12,14 +12,10 @@
 
   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, rust-nixpkgs, ... }:
+    { self, nixpkgs, ... }:
     let
       supportedSystems = [
         "x86_64-linux"
@@ -40,10 +36,7 @@
             inherit system;
             overlays = [ self.overlays.default ];
           };
-          # 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; };
+          mcpPackages = import ./packages { inherit pkgs; };
         in
         mcpPackages // {
           default = pkgs.symlinkJoin {
@@ -60,8 +53,7 @@
             inherit system;
             overlays = [ self.overlays.default ];
           };
-          rustPkgs = import rust-nixpkgs { inherit system; };
-          mcpPackages = import ./packages { inherit pkgs rustPkgs; };
+          mcpPackages = import ./packages { inherit pkgs; };
         in
         {
           default = pkgs.mkShell {