This way we can get logs also in case we cancelled the job due to a test
hanging indefinitely (which is happening from time to time to the Xcode
test).
Because of https://github.com/actions/runner/issues/2009 the deeply
nested action cache was failing to save the cache in the post run phase.
For the moment we just avoid the nesting with a copy-pasted action
snippet.
Previously the cache would become stale. Now the same incremental
cache mechanism in use for the QL cache is adopted (and factored out
in a separate action).
Namely, pushes on main will populate the cache using the commit hash as
key, while PRs will try to use the cache of their merge base, read-only.
To avoid the cache growing out of control, a simple cache eviction is
done on pushes.
* A unique workflow file has been created merging all `swift-*.yml`
workflows
* Change filtering at job level was added using [dorny/paths-filter][1]
* only one build of the extractor is made, and then shared via cache
(not as an artifact because of [this longstading issue][2])
* integration tests are now run on on macOS
* qltests are not run any more on macOS to cut on feedback time
* autobuilder tests were moved to the macOS build step to avoid loading
bazel twice
[1]: https://github.com/dorny/paths-filter#examples
[2]: https://github.com/actions/upload-artifact/issues/38
