nixos/testing: only create nixos-run-vms for nixos-build-vms(8)

This is the case when the test-script is empty. `nixos-build-vms(8)` is
primarily supposed to be used as tool to test changes or to reproduce
bugs (IMHO) where "just spinning up a few VMs" is the primary use-case.

In the ongoing discussion about these changes[1] it was suggested to
only expose it when needed (i.e. in the case I described above) to keep
the API surface as slim as possible.

[1] https://github.com/NixOS/nixpkgs/pull/133675#discussion_r688112485

nixos/lib/testing-python.nix

@@ -187,11 +187,13 @@ rec {
           --set testScript "$out/test-script" \
           --set vlans '${toString vlans}'
 
-        ln -s ${testDriver}/bin/nixos-test-driver $out/bin/nixos-run-vms
-        wrapProgram $out/bin/nixos-run-vms \
-          --set startScripts "''${vmStartScripts[*]}" \
-          --set testScript "${pkgs.writeText "start-all" "start_all(); join_all();"}" \
-          --set vlans '${toString vlans}'
+        ${lib.optionalString (testScript == "") ''
+          ln -s ${testDriver}/bin/nixos-test-driver $out/bin/nixos-run-vms
+          wrapProgram $out/bin/nixos-run-vms \
+            --set startScripts "''${vmStartScripts[*]}" \
+            --set testScript "${pkgs.writeText "start-all" "start_all(); join_all();"}" \
+            --set vlans '${toString vlans}'
+        ''}
       '');
 
   # Make a full-blown test