BlueJ doesn't start on nixOS

[koppor]

❯ ./gg.cmd run gh/k-pet-group/BlueJ-Greenfoot

thread 'main' (163459) panicked at src/main.rs:509:18:
called `Result::unwrap()` on an `Err` value: "Error: Unable to find executable for custom_command. The tool may not be properly installed or the binary name doesn't match expected patterns."
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I cannot read the error message.

Maybe, gg cmd downloads .deb, but it should download the jar? (refs #213)