The goal of this change is to run a trimmed-down CI on PRs by default,
but allows opt-in the full CI as necessary.
### Specification
The CI workflow runs in “quick” mode if it was triggered from a pull
request, and that pull request does not have the `full-ci` label set.
In “quick” mode the build matrix contains fewer jobs. At the moment
only:
* Linux-release, to get the PR releases.
In non-quick mode everything should be as before.
### Implementation notes
I created a `configure` job that combines all the previous `set-` jobs,
I guess this is faster than firing up separate jobs.
The matrix is calculated in this job; this seems to be the cleanest way
to get a dynamic matrix going (experiments using `exclude` failed). The
downside is that the matrix is now in JSON rather than Yaml syntax. The
upside is that we can (later) make it’s calculation simpler, e.g. set
default `shell` values etc.
I was not able to make it so that CI runs when the `full-ci` label is
added, but don’t do anything otherwise. I think it can be done with
another workflow listening to `labeled` and then triggering this one,
but let’s do that separately. For now, add the label and then push (or
close and reopen).
The checks
```
if: matrix.build-stage2 || matrix.check-stage3
if: matrix.check-stage3
```
were dead code, we did not have these fields in the matrix anymore, so I
replaced them with
```
if: matrix.test-speedcenter
```
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