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.nix

@@ -89,7 +89,7 @@ name: testConfig:
   vmWithoutTests = (pkgs.nixos ({ config, ... }: {
     imports = [
       testConfig
-      "${toString pkgs.path}/nixos/modules/virtualisation/qemu-vm.nix"
+      (pkgs.path + "/nixos/modules/virtualisation/qemu-vm.nix")
     ];
     virtualisation.graphics = false;
     services.getty.autologinUser = "root";