Improve GitHub Actions workflow

[djdv]

What we have today works, but probably isn't ideal; so I won't be prioritizing this myself.
But if anyone wants to critique it and point out where we're not using good practice, please do.

Concerns:
We're utilizing multiple jobs, which means setting up multiple machines when we probably don't have to.
For example, we don't need 2 machines where one runs the tests and another runs the build, the same machine could just do both in sequence, or better - in parallel.

The conditions for jobs like 'release' are going to be skipped on the majority of runs.
It'd be better if this just didn't show up at all unless it was needed, rather than exist and get ignored.
It doesn't serve a purpose other than being chaff in the UI+logs when it's not actually executed.

It's unclear what the preferred method is for various conditionals, and the syntax + feature set of Actions has been and still is changing; but we could probably eliminate some uses of if by just better utilizing runs-on so that we don't have to query what OS we're on as much and instead just have straight paths for each system to execute rather than a single task with branching paths.

inputs and outputs, as well as environment variable semantics has changed multiple times on the GitHub side.
I think I updated all of these, however, it's possible there's newer|better ways to pass data between tasks, jobs, etc. that we're not doing properly today.