Adjust retrieval of build artifacts to match current filenames.

This fixes a regression introduced by
commit:787eb59e, which renamed the artifacts but
did not adjust the code that retrieves them from the build VM.

Rakefile

@@ -314,7 +314,7 @@ end
 def list_artifacts
   user = vagrant_ssh_config('User')
   stdout = capture_vagrant_ssh("find '/home/#{user}/amnesia/' -maxdepth 1 " +
-                                        "-name 'tails-*.iso*' -o -name 'tails-*.img'").first
+                                        "-name 'tails-amd64-*'").first
   stdout.split("\n")
 rescue VagrantCommandError
   return Array.new