Request: GitHub Action artifact support

[jamesbraza]

Since shareable URLs only last two weeks, I am now persisting my promptfoo outputs via https://github.com/actions/upload-artifact.

It would be nice to either:

• Support artifacts directly in this action
• Support output arg in this action
• Allow promptfoo eval --output to pull from an environment variable PROMPTFOO_OUTPUT

[jamesbraza]

Actually, it looks like by default this action creates an output.json: https://github.com/promptfoo/promptfoo-action/blob/v1/src/main.ts#L124

I think this issue can alternately be closed by just documenting that in the README.md (and optionally making that path configurable)