nixos/tests/netboot: Fix evaluation error

Regression introduced by dfe608c8a2.

The commit turns the two arguments into one attrset argument so we need
to adapt that to use the new calling convention.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/tests/boot.nix

@@ -64,11 +64,14 @@ in {
         '';
         destination = "/boot.ipxe";
       };
-      ipxeBootDir = pkgs.symlinkJoin "ipxeBootDir" [
+      ipxeBootDir = pkgs.symlinkJoin {
+        name = "ipxeBootDir";
+        paths = [
           config.system.build.netbootRamdisk
           config.system.build.kernel
           ipxeScriptDir
         ];
+      };
     in
       makeTest {
         name = "boot-netboot";