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/nix.py

@@ -1,10 +1,42 @@
 import os
 
+from .dirs import flake_registry, unfree_nixpkgs
+
 
 def nix_shell(packages: list[str], cmd: list[str]) -> list[str]:
-    flake = os.environ.get("CLAN_FLAKE")
-    # in unittest we will have all binaries provided
-    if flake is None:
+    # we cannot use nix-shell inside the nix sandbox
+    # in our tests we just make sure we have all the packages
+    if os.environ.get("IN_NIX_SANDBOX"):
         return cmd
-    wrapped_packages = [f"path:{flake}#{p}" for p in packages]
-    return ["nix", "shell"] + wrapped_packages + ["-c"] + cmd
+    wrapped_packages = [f"nixpkgs#{p}" for p in packages]
+    return (
+        [
+            "nix",
+            "shell",
+            "--extra-experimental-features",
+            "nix-command flakes",
+            "--flake-registry",
+            str(flake_registry()),
+        ]
+        + wrapped_packages
+        + ["-c"]
+        + cmd
+    )
+
+
+def unfree_nix_shell(packages: list[str], cmd: list[str]) -> list[str]:
+    if os.environ.get("IN_NIX_SANDBOX"):
+        return cmd
+    return (
+        [
+            "nix",
+            "shell",
+            "--extra-experimental-features",
+            "nix-command flakes",
+            "-f",
+            str(unfree_nixpkgs()),
+        ]
+        + packages
+        + ["-c"]
+        + cmd
+    )