core: generate bogus gemfile so that a random lockfile isn't loaded

lib/vagrant/bundler.rb

@@ -205,7 +205,7 @@ def with_isolated_gem
       # native extensions because it causes all sorts of problems.
       old_rubyopt = ENV["RUBYOPT"]
       old_gemfile = ENV["BUNDLE_GEMFILE"]
-      ENV["BUNDLE_GEMFILE"] = nil
+      ENV["BUNDLE_GEMFILE"] = Tempfile.new("vagrant-gemfile").path
       ENV["RUBYOPT"] = (ENV["RUBYOPT"] || "").gsub(/-rbundler\/setup\s*/, "")
 
       # Set the GEM_HOME so gems are installed only to our local gem dir