deb: remove hard dependency on distro-provided Nix

The Debian Nix package causes flake builds to fail inside the test and
container environment due to sandboxing and patched Nix behavior.

To ensure consistent behaviour across all distributions and align
container logic with production logic, pkgmgr now relies on its own
`init-nix.sh` bootstrap script instead of the distro’s `nix` package.

Dropping `Depends: nix` guarantees that both Debian containers and real
Debian systems install and initialize Nix via the upstream installer,
matching the behaviour on Arch, Fedora, and Ubuntu.

https://chatgpt.com/share/693ab9bf-e6ac-800f-83ba-a4abd1bfe407

packaging/debian/control

@@ -9,7 +9,7 @@ Homepage: https://github.com/kevinveenbirkenbach/package-manager
 
 Package: package-manager
 Architecture: any
-Depends: nix, ${misc:Depends}
+Depends: ${misc:Depends}
 Description: Wrapper that runs Kevin's package-manager via Nix flake
  This package provides the `pkgmgr` command, which runs Kevin's package
  manager via a local Nix flake