add compatibility with Nix PR #6530 (Source tree abstraction)

Avoid adding flake resource paths to the store (via string
interpolation).
This reduces performance and can lead to modules getting imported
twice, once through a local path and once through a store path.

This might not be needed in a future Nix release, in which case we can
revert this.

test/lib/make-test-vm.nix

@@ -1,6 +1,6 @@
 pkgs:
 let
-  pythonTesting = import "${toString pkgs.path}/nixos/lib/testing-python.nix" {
+  pythonTesting = import (pkgs.path + "/nixos/lib/testing-python.nix") {
     system = pkgs.stdenv.hostPlatform.system;
     inherit pkgs;
   };