Fix attached runtime to not rely on Mix

lib/livebook/runtime/erl_dist/runtime_server.ex

@@ -855,14 +855,21 @@ defmodule Livebook.Runtime.ErlDist.RuntimeServer do
     # We propagate Mix.install/2 project dir in the transient state,
     # so that future runtimes can set it as the starting point for
     # Mix.install/2
-    if dir = state.mix_install_project_dir == nil && Mix.install_project_dir() do
+    if dir = state.mix_install_project_dir == nil && install_project_dir() do
       send(state.owner, {:runtime_transient_state, %{mix_install_project_dir: dir}})
       %{state | mix_install_project_dir: dir}
     else
       state
     end
   end
 
+  defp install_project_dir() do
+    # Make sure Mix is loaded (it may not be in case of attached runtime)
+    if Code.ensure_loaded?(Mix) do
+      Mix.install_project_dir()
+    end
+  end
+
   defp scan_binding_async(_ref, %{scan_binding: nil} = info, _state), do: info
 
   # We wait for the current scanning to finish, this way we avoid