replace environment variable with nixpkgs directory

In this directory we generate all the files that we need to load nixpkgs.
This seems more robust than all those environment variables that may or not may be set.

pkgs/clan-cli/clan_cli/dirs.py

@@ -24,3 +24,19 @@ def user_config_dir() -> Path:
         return Path(os.path.expanduser("~/Library/Application Support/"))
     else:
         return Path(os.getenv("XDG_CONFIG_HOME", os.path.expanduser("~/.config")))
+
+
+def module_root() -> Path:
+    return Path(__file__).parent
+
+
+def flake_registry() -> Path:
+    return module_root() / "nixpkgs" / "flake-registry.json"
+
+
+def nixpkgs() -> Path:
+    return (module_root() / "nixpkgs" / "path").resolve()
+
+
+def unfree_nixpkgs() -> Path:
+    return module_root() / "nixpkgs" / "unfree"