Use system-features from the Nix conf in the default machine file

Fix #936

t/jobs/default-machine-file.nix

@@ -0,0 +1,9 @@
+with import ./config.nix;
+{
+  requireExperimentalFeatures =
+    mkDerivation {
+      name = "empty-dir";
+      builder = ./empty-dir-builder.sh;
+      requiredSystemFeatures = [ "test-system-feature" ];
+    };
+}