do less string coercion in test modules in case that reduces copies

idk honestly but these tests take sooo long I'm paranoid

tests/dynix-vm-configuration.nix

@@ -1,6 +1,6 @@
 { pkgs, lib, modulesPath, ... }:
 let
-  moduleList = import "${modulesPath}/module-list.nix";
+  moduleList = import (modulesPath + "/module-list.nix");
 
   dynixFromSearchPath = let
     res = builtins.tryEval <dynix>;
@@ -8,7 +8,7 @@ let
 in
 {
   imports = [
-    "${modulesPath}/testing/test-instrumentation.nix"
+    (modulesPath + "/testing/test-instrumentation.nix")
   ] ++ lib.concatLists [
     dynixFromSearchPath
     moduleList
@@ -26,7 +26,7 @@ in
   nix = {
     package = pkgs.lixPackageSets.latest.lix;
     nixPath = [
-      "nixpkgs=${pkgs.path}"
+      ("nixpkgs=" + pkgs.path)
       "/nix/var/nix/profiles/per-user/root/profile/share/nixos/modules"
     ];