factor out GHA-specific defaults

[maxheld83]

This is a nit, so apologies in advance and please close ruthlessly at your pleasure, but:

I'd suggest to move out all the defaults related to GitHub Actions out of this repo (and into https://github.com/DeterminateSystems/ci?).

For example, the environment variable $GITHUB_OUTPUT is not a concern for flake-iter proper, nor are the default mappings like "x86_64-linux": "ubuntu-latest".

Advantages:

• makes the CLI a bit cleaner (i.e. flake-iter systems just returning to stdout)
• easier to debug/reason about (e.g., in CI fails with empty matrix vector ci#19, I got confused whether this was a DeterminateSystems/ci or DeterminateSystems/flake-iter issue)
• easier to update if GHA spec/api changes
• easier to add other CI vendors

I'd be happy to try my hand at this with a PR, but wanted to ask first whether I even got this right in terms of design.