Allow diff-system to fail

When upgrading from older systems with nix 2.3.x the command is not yet
understood. Since it is purely informational a failure can be safely
ignored.

modules/autoupdate.nix

@@ -68,7 +68,7 @@
       text = ''
         if [ -e /run/current-system ] && [ -e $systemConfig ]; then
           echo System package diff:
-          ${config.nix.package}/bin/nix store diff-closures /run/current-system $systemConfig
+          ${config.nix.package}/bin/nix store diff-closures /run/current-system $systemConfig || true
         fi
       '';
     };