Support both bzlmod and nix in start script

[jonathanlking]

Is your feature request related to a problem? Please describe.

The start script doesn't support both --use-nix and --with-bzlmod=true.

Describe the solution you'd like
The script to support it!

Describe alternatives you've considered

Additional context

@LiamGoodacre and I have been working on an example that uses both bzlmod and nix at https://github.com/jonathanlking/rules-haskell-bzlmod-nix, which I intend to open a PR to update the start script to output.