Autogenerate `s3_destination` parameter value by default in `Tracker.declare_output`

[dmcc]

s3_destination could be autogenerated which might make it slightly easier to use (and slightly more predictable S3 structures).

Also relates to a potential Action refactor, roughly demonstrated with this snippet: https://gist.github.com/dmcc/830ca0712e399770f652c042c27958b4