system/pc: re-enable QEMU binfmt

it seems like the relevant issues have been fixed:
- https://github.com/NixOS/nixpkgs/issues/221056 (compilation time)
- https://github.com/NixOS/nixpkgs/issues/160300 (chroot not working)

system/profile/pc.nix

@@ -14,9 +14,7 @@ in
   config = lib.mkIf cfg.enable {
     nixfiles.profile.base.enable = lib.mkDefault true;
 
-    # disabling this for now; it doesn't work in chroot yet (the only reason i
-    # want it) and it takes forever to compile.
-    # nixfiles.binfmt.enable = lib.mkDefault true;
+    nixfiles.binfmt.enable = lib.mkDefault true;
 
     # networking.hostName = "nixos"; # Define your hostname.
     # Pick only one of the below networking options.