I'm developing a Django app, which works fine so far: Railpack detects Python and uv and finds the app. But now I'm trying to add a Node build step for some Javascript. My railway.toml has

buildCommand = "uv sync --frozen && cd frontend && npm install && npm run build && cd .. && uv run python manage.py collectstatic --noinput"

and this is failing with sh: 1: npm: not found

I see in the railpack documentation that I could set RAILPACK_PACKAGES to include extra mise packages, but where do I set this variable?