Add artifact support for multi-job workflows

[jmealo]

Reported in #187

It appears if a workflow contains multiple jobs, the post-run job will fail as the log files are no longer present in the file system.

In order to address this we'd need to use actions/upload-artifact and actions/download-artifact to pass the log files between jobs.

I haven't researched whether this is something that can be done declaratively by end-users or if we need to make changes to the post-run.js code to support this.

[themightychris]

@jmealo IME it's pretty common for actions to leave it to the workflow author to insert actions/upload-artifact or actions/download-artifact steps themselves if they want to pass artifacts between jobs, I don't think I've seen many/any actions that build in that functionality. Rather we just document and/or put in an output value the path where our action leaves useful files

I think we'd want our responsibility to end at providing an example in the README showing how to use background-action together with actions/upload-artifact and actions/download-artifact to carry a log across jobs