title: Is it possible to trigger Packer's post-processor skipping the build step?
body: <p>Let's asume I have a Packer file named <code>windows_10.json</code>. I ran <code>packer build windows_10.json</code> and it takes 3 to 4 hours to build a VirtualBox image of Windows 10. The build process finishes successfully and creates a VirtualBox image consisting of a <code>.ovf</code> and a <code>.vmdk</code> file compressed in a <code>.zip</code> file. Let's further asume that I have forgotten to add the code below for a vagrant post-processor to my <code>windows_10.json</code> file which should produce a Vagrant box:</p>
  
  <p><code>
  "post-processors": [
      {
        "type": "vagrant",
        "only": ["virtualbox-iso"],
        "keep_input_artifact": true,
        "output": "windows_10_{{.Provider}}.box",
        "vagrantfile_template": "vagrantfile-windows_10.template"
      }
  </code></p>
  
  <p>Now my question is: After adding the above code to <code>windows_10.json</code> file can I instruct Packer to do the post-process step ONLY and create a Vagrant box skipping the long build process? I don´t want to wait 3 to 4 hours again but instead continue with the creation of the Vagrant box.</p>