nix: empty flake.nix

This commit slightly improves how batsched can be imported in experiments designed as Nix flakes.

This enables batsched source code to be fetched with full flake traceability (lock file),
which can be useful for fetching non-public Batsim copies.

Usage is very limited with this empty version though, a proper flake setup should override this file.

flake.nix

+{
+  outputs = {...}: { };
+}