Run packer as a build step

I'm using some secret text/file injectors that insert pem files from from a credential provider. Unfortunately these secret files do not persist into the post-build steps. Would it be possible to run packer as a build step rather than a post-build/publisher step?

Start by tracing how the plugin currently invokes Packer as a post-build or publisher step and how Jenkins build-step extension points handle credential-injected files. Verify the behavior with PEM files from a credential provider; done means Packer can run as a build step and those files remain available to later post-build steps.

Written by the indexing model from the issue text.