nixos/system: make switch-to-configuration script pure.

Fixes #28443

Fixed few invocations to `systemctl` to have an absolute path. Additionally add
LOCALE_ARCHIVE so that perl stops spewing warning messages.

nixos/tests/switch-test.nix

@@ -0,0 +1,25 @@
+# Test configuration switching.
+
+import ./make-test.nix ({ pkgs, ...} : {
+  name = "switch-test";
+  meta = with pkgs.stdenv.lib.maintainers; {
+    maintainers = [ gleber ];
+  };
+
+  nodes = {
+    machine = { config, lib, pkgs, ... }: {
+      users.mutableUsers = false;
+    };
+    other = { config, lib, pkgs, ... }: {
+      users.mutableUsers = true;
+    };
+  };
+
+  testScript = {nodes, ...}: let
+    originalSystem = nodes.machine.config.system.build.toplevel;
+    otherSystem = nodes.other.config.system.build.toplevel;
+  in ''
+    $machine->succeed("env -i ${originalSystem}/bin/switch-to-configuration test | tee /dev/stderr");
+    $machine->succeed("env -i ${otherSystem}/bin/switch-to-configuration test | tee /dev/stderr");
+  '';
+})