cli: resolve CLAN_NIXPKGS on every invocation

this makes mocking easier

pkgs/clan-cli/clan_cli/nix.py

@@ -1,10 +1,9 @@
 import os
 
-CLAN_NIXPKGS = os.environ.get("CLAN_NIXPKGS")
-
 
 def nix_shell(packages: list[str], cmd: list[str]) -> list[str]:
+    nixpkgs = os.environ.get("CLAN_NIXPKGS")
     # in unittest we will have all binaries provided
-    if CLAN_NIXPKGS is None:
+    if nixpkgs is None:
         return cmd
-    return ["nix", "shell", "-f", CLAN_NIXPKGS] + packages + ["-c"] + cmd
+    return ["nix", "shell", "-f", nixpkgs] + packages + ["-c"] + cmd