nix: make hotpath an opt-in command (#283)

To verify:
```
nix build .#default 
nix path-info --recursive .#default | rg -i hotpath
```

`nix flake show` output:
```
├───apps
│   ├───aarch64-darwin
│   │   ├───default: app
│   │   ├───hotpath: app
│   │   └───nix-build-push: app
│   ├───aarch64-linux omitted (use '--all-systems' to show)
│   ├───x86_64-darwin omitted (use '--all-systems' to show)
│   └───x86_64-linux omitted (use '--all-systems' to show)
├───checks
│   ├───aarch64-darwin
│   │   ├───clippy: CI test
│   │   ├───fmt: CI test
│   │   └───npm-package: CI test
│   ├───aarch64-linux omitted (use '--all-systems' to show)
│   ├───x86_64-darwin omitted (use '--all-systems' to show)
│   └───x86_64-linux omitted (use '--all-systems' to show)
├───devShells
│   ├───aarch64-darwin
│   │   ├───default: development environment
│   │   ├───manual: development environment
│   │   └───release: development environment
│   ├───aarch64-linux omitted (use '--all-systems' to show)
│   ├───x86_64-darwin omitted (use '--all-systems' to show)
│   └───x86_64-linux omitted (use '--all-systems' to show)
└───packages
    ├───aarch64-darwin
    │   ├───default: package
    │   ├───hotpath: package
    │   ├───manual: package
    │   ├───nix-build-push: package
    │   ├───npm-package: package
    │   └───release: package
    ├───aarch64-linux omitted (use '--all-systems' to show)
    ├───x86_64-darwin omitted (use '--all-systems' to show)
    └───x86_64-linux omitted (use '--all-systems' to show)
```