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/flakes/flake.nix

@@ -14,7 +14,7 @@
         # Import the secure-node preset, an opinionated config to enhance security
         # and privacy.
         #
-        # "${nix-bitcoin}/modules/presets/secure-node.nix"
+        # (nix-bitcoin + "/modules/presets/secure-node.nix")
 
         {
           # Automatically generate all secrets required by services.

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;

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;

pkgs/spark-wallet/composition.nix

@@ -5,7 +5,7 @@
   }, system ? builtins.currentSystem, nodejs ? pkgs."nodejs-14_x"}:
 
 let
-  nodeEnv = import "${toString pkgs.path}/pkgs/development/node-packages/node-env.nix" {
+  nodeEnv = import (pkgs.path + "/pkgs/development/node-packages/node-env.nix") {
     inherit (pkgs) stdenv lib python2 runCommand writeTextFile writeShellScript;
     inherit pkgs nodejs;
     libtool = if pkgs.stdenv.isDarwin then pkgs.darwin.cctools else null;

test/clightning-replication.nix

@@ -6,12 +6,12 @@ in
 import "${nixpkgs}/nixos/tests/make-test-python.nix" ({ pkgs, ... }:
 with pkgs.lib;
 let
-  keyDir = "${nixpkgs}/nixos/tests/initrd-network-ssh";
+  keyDir = nixpkgs + "/nixos/tests/initrd-network-ssh";
   keys = {
-    server = "${keyDir}/ssh_host_ed25519_key";
-    client = "${keyDir}/id_ed25519";
-    serverPub = readFile "${keys.server}.pub";
-    clientPub = readFile "${keys.client}.pub";
+    server = keyDir + "/ssh_host_ed25519_key";
+    client = keyDir + "/id_ed25519";
+    serverPub = readFile (keys.server + ".pub");
+    clientPub = readFile (keys.client + ".pub");
   };
 
   clientBaseConfig = {

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;
   };

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";