examples: use relative paths to example files

This ensures that the examples files from the current example dir are used.
Previously, the default examples from the nix-bitcoin source were used.

examples/deploy-qemu-vm.sh

@@ -38,7 +38,7 @@ nix-build --out-link $tmpDir/vm - <<EOF
 (import <nixpkgs/nixos> {
   configuration = {
     imports = [
-      <nix-bitcoin/examples/configuration.nix>
+      <configuration.nix>
       <nix-bitcoin/modules/secrets/generate-secrets.nix>
     ];
     virtualisation.graphics = false;