change is_apt method

Author: David Maloney

lib/msf/ui/console/command_dispatcher/core.rb

@@ -3067,7 +3067,7 @@ def is_metasploit_debian_package_installed
 
   # Determines if this is an apt-based install
   def is_apt
-    File.exists?(File.expand_path(File.join(msfbase_dir, '.apt')))
+    File.exists?(File.expand_path(File.join(Msf::Config.install_root, '.apt')))
   end
 
   # Determines if we're a Metasploit Pro/Community/Express