Remove references to Root SBDs as inputs, instead use `_name` Reserved Definition

## Background

The entrypoint input `inputs.root-sbd` can be inferred from the `_name` reserved definition now. This means that we can git rid of the propogation of a lot of those variables and simplify the overall pipeline. 

A similar consideration can be made for `inputs.name`, which could probably be inferred from `github.repository`, or dropped entirely in favour of `_name`. 
