Start generating separate "local" jobs instead of using a build matrix

[fasterthanlime]

We're running into GHA limitations: for example we cannot template steps.0.shell, see #1509 (comment)

This also means we cannot opt into bash (as opposed to sh) when installing dist itself (or cargo auditable, for that matter).

Part of #1509 become meaningful again (the whole GhaRunStep type) once we start doing some of the templating in dist itself.

It's also going to become a lot clearer what the CI pipeline is actually going to do, by just looking at the generated release.yml, rather than the actual steps being "hidden" in an artifact generated by plan.