python3 missing at runtime (Nixpacks)

Question

I'm using Nixpacks and added python3 to the nixPkgs in the setup phase. It looks like it's installed, but when my app runs, it still throws /bin/sh: 1: python3: not found. Any idea why python3 isn’t available at runtime, even though it's in the Nixpacks config?
```
[phases.setup]
nixPkgs = ["...", "python3"]

providers = ["...", "python"]

[start]
cmd = "PATH=$PATH:/root/.nix-profile/bin npm start"
```