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/tests/conftest.py

@@ -6,6 +6,8 @@ from typing import Generator
 
 import pytest
 
+from clan_cli.dirs import nixpkgs
+
 sys.path.append(os.path.join(os.path.dirname(__file__), "helpers"))
 
 pytest_plugins = [
@@ -27,15 +29,8 @@ def monkeymodule() -> Generator[pytest.MonkeyPatch, None, None]:
         yield mp
 
 
-# fixture for the example flake located under ./example_flake
-# The flake is a template that is copied to a temporary location.
-# Variables like __CLAN_NIXPKGS__ are replaced with the value of the
-# CLAN_NIXPKGS environment variable.
 @pytest.fixture(scope="module")
 def machine_flake(monkeymodule: pytest.MonkeyPatch) -> Generator[Path, None, None]:
-    CLAN_NIXPKGS = os.environ.get("CLAN_NIXPKGS", "")
-    if CLAN_NIXPKGS == "":
-        raise Exception("CLAN_NIXPKGS not set")
     template = Path(__file__).parent / "machine_flake"
     # copy the template to a new temporary location
     with tempfile.TemporaryDirectory() as tmpdir_:
@@ -49,7 +44,7 @@ def machine_flake(monkeymodule: pytest.MonkeyPatch) -> Generator[Path, None, Non
         # provided by get_clan_flake_toplevel
         flake_nix = flake / "flake.nix"
         flake_nix.write_text(
-            flake_nix.read_text().replace("__CLAN_NIXPKGS__", CLAN_NIXPKGS)
+            flake_nix.read_text().replace("__NIXPKGS__", str(nixpkgs()))
         )
         # check that an empty config is returned if no json file exists
         monkeymodule.chdir(flake)