Note to contributors: Two CI approaches, two baseline files

[dg0yt]

There are effectively two CI jobs for PRs:

• The traditional CI tests only one configuration of each port. For PRs, it tests all ports which have a changed ABI hash and no matching binary cache entry ("cone of destruction"). It is controlled by scripts/ci.baseline.txt.
• The feature CI tests each supported feature and the combination of all supported features for ports which have been modified. It needs the more detailed scripts/ci.features.baseline.txt.

If a port generally fails in CI (i.e. due to missing SDK), it will have similar notes in both files. But one file cannot replace the other because they test different configurations. (Even for the same feature set, the configurations of the dependencies can differ.)

So when dealing with CI errors in port updates, check which filenames are suggested to be updated. The new feature check uncovers a number of problems which were previously hidden.