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/vm-config.nix

@@ -3,7 +3,7 @@
   # Disable the hardened preset to improve VM performance
   disabledModules = [ <nix-bitcoin/modules/presets/hardened.nix> ];
 
-  imports = [ "${modulesPath}/virtualisation/qemu-vm.nix" ];
+  imports = [ (modulesPath + "/virtualisation/qemu-vm.nix" ];
 
   config = {
     virtualisation.graphics = false;