Set status target URL to HALT file contents

[pbrisbin]

If the HALT file is used for details about the reason, we should link to it from the PR status.

This will interact with the inputs.status-target-url defaults, we'll need to check message.wasOrigionallyEmpty, and we'll need need the github.repository context to construct a URL to the HALT file contents page.

[pbrisbin]

This actually isn't too bad right now since we add the HALT file contents do a workflow summary, then the status-target-url links there (by default). It may not be worth the complexity.