use xvfb-run to create headless anki if no running instance is available

flake.nix

@@ -20,7 +20,7 @@
     toolchain = fenix.packages.x86_64-linux.minimal.toolchain;
   in {
     devShells.x86_64-linux.default = pkgs.mkShell {
-      buildInputs = [toolchain];
+      buildInputs = [toolchain pkgs.xvfb-run];
       shellHook = ''
         export RUSTUP_TOOLCHAIN=${toolchain}
       '';
@@ -30,7 +30,12 @@
         cargo = toolchain;
         rustc = toolchain;
       }).buildPackage {
+        nativeBuildInputs = [pkgs.makeWrapper];
         src = ./.;
+        postInstall = ''
+          wrapProgram $out/bin/anki-cli \
+          --prefix PATH : ${pkgs.xvfb-run}/bin
+        '';
       };
   };
 }