put hello and tree in the agent-ephemeral-rootfs vm closure

nix/vm-tests/agent-ephemeral-rootfs.nix

@@ -72,6 +72,12 @@ pkgs.testers.runNixOSTest {
         ../host-modules/hive-priv.nix
       ];
       virtualisation.memorySize = 2048;
+      # The script `nix-env -i`s these store paths inside the container, which
+      # shares the host's store; the VM has no substituter to fetch them.
+      virtualisation.additionalPaths = [
+        pkgs.hello
+        pkgs.tree
+      ];
       # The agent bridge hive-priv names in the container's conf.
       networking.bridges.${bridge}.interfaces = [ ];
       networking.interfaces.${bridge}.ipv4.addresses = [