nixos/binfmt: fix extension for Windows binaries

Extension based matching for Windows targets define it '.exe' but kernel
documentation explicitly states it should be passed "without the .".

From https://docs.kernel.org/admin-guide/binfmt-misc.html
> * magic:
>   [...] If you chose filename extension matching, this is the
>   extension to be recognised (without the ., the \x0a specials are not
>   allowed).

nixos/modules/system/boot/binfmt.nix

@@ -134,11 +134,11 @@ let
       mask = ''\xff\xff\xff\xff'';
     };
     x86_64-windows = {
-      magicOrExtension = ".exe";
+      magicOrExtension = "exe";
       recognitionType = "extension";
     };
     i686-windows = {
-      magicOrExtension = ".exe";
+      magicOrExtension = "exe";
       recognitionType = "extension";
     };
   };