ci: use latest wayland-protocol for all test cases

Fixes missing protocols on old nixpkgs versions

ci/matrix.nix

@@ -2,7 +2,10 @@
   qtver,
   compiler,
 }: let
-  nixpkgs = (import ./nix-checkouts.nix).${builtins.replaceStrings ["."] ["_"] qtver};
+  checkouts = import ./nix-checkouts.nix;
+  nixpkgs = checkouts.${builtins.replaceStrings ["."] ["_"] qtver};
   compilerOverride = (nixpkgs.callPackage ./variations.nix {}).${compiler};
-  pkg = (nixpkgs.callPackage ../default.nix {}).override compilerOverride;
+  pkg = (nixpkgs.callPackage ../default.nix {}).override (compilerOverride // {
+    wayland-protocols = checkouts.latest.wayland-protocols;
+  });
 in pkg

ci/nix-checkouts.nix

@@ -7,10 +7,12 @@ let
     url = "https://github.com/nixos/nixpkgs/archive/${commit}.tar.gz";
     inherit sha256;
   }) {};
-in {
+in rec {
   # For old qt versions, grab the commit before the version bump that has all the patches
   # instead of the bumped version.
 
+  latest = qt6_9_0;
+
   qt6_9_0 = byCommit {
     commit = "546c545bd0594809a28ab7e869b5f80dd7243ef6";
     sha256 = "0562lbi67a9brfwzpqs4n3l0i8zvgla368aakcy5mghr7ps80567";