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.

examples/qemu-vm/minimal-vm.nix

@@ -13,13 +13,13 @@ rec {
     QEMU_OPTS="-smp $(nproc) -m 1500" ${vm}/bin/run-*-vm
   '';
 
-  vm = (import "${nixpkgs}/nixos" {
+  vm = (import (nixpkgs + "/nixos") {
     inherit system;
     configuration = { config, lib, modulesPath, ... }: {
       imports = [
         nix-bitcoin.nixosModules.default
-        "${nix-bitcoin}/modules/presets/secure-node.nix"
-        "${modulesPath}/virtualisation/qemu-vm.nix"
+        (nix-bitcoin + "/modules/presets/secure-node.nix")
+        (modulesPath + "/virtualisation/qemu-vm.nix")
       ];
 
       virtualisation.graphics = false;