zerotier: move nix_shell to external package

pkgs/clan-cli/clan_cli/nix.py

@@ -0,0 +1,10 @@
+import os
+
+CLAN_NIXPKGS = os.environ.get("CLAN_NIXPKGS")
+
+
+def nix_shell(packages: list[str], cmd: list[str]) -> list[str]:
+    # in unittest we will have all binaries provided
+    if CLAN_NIXPKGS is None:
+        return cmd
+    return ["nix", "shell", "-f", CLAN_NIXPKGS] + packages + ["-c"] + cmd