Motivation for this change

mu can in some scenarios use quite a bit of memory and no longer
supports a runtime parameter to adjust the memory use.

This change allows one to override the batch size if needed.

With few reverse dependencies, it shouldn't be an issue to compile this
locally but by default we don't change anything and stick to upstream.

ref: djcb/mu#1653

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.