systemd-boot: Boot the default entry by default

Since I never use previous generations, booting the newest entry by
default seems ideal. In the case that something is broken, it should be
possible to return to the menu by pressing space at boot.

flake.nix

@@ -60,6 +60,7 @@
               editor = false;
             };
 
+            timeout = 0;
             efi.canTouchEfiVariables = true;
           };