Expose default artifact location information through a workflow variable

# Summary

Key-only artifacts are really cool -- I love the way they can decouple the artifact location configuration from the workflows and templates that I use. Previously I had employed a workflow parameter I called `bucket-name` throughout all of my workflows which I would then call for every single bucket/key in a repository artifact.

However, I also had a few other use cases for this bucket name, mostly involving producing links to where the data is stored. If key-only artifacts mean I can infer the bucket, it would also be useful for these purposes for them to be exposed through a workflow variable, like `{{workflow.bucket-name}}` (or a more complicated expression with more information in addition to the bucket name). That way I can make sure that the link I provide matches the actual location where the artifacts are stored.

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The issue names no files, tests, or entry points. Start by tracing how key-only artifact locations are resolved and how workflow variables are evaluated; done means a workflow can expose the resolved default artifact location so generated links match where artifacts are stored.

Written by the indexing model from the issue text.