[fix] CI: prevent race condition
`checker.yml` and `integration.yml` are the only workflows that are currently safe to be executed simultaneously, the others present a risk that the order of completion may not be expected. The ones that are chained from `integration.yml` can be called as many times as `integration.yml` workflows are running at that moment, the same with the trigger "workflow_dispatch".
This can be fatal for workflows like `container.yml` that use a centralized cache to store and load the candidate images in a common tag called "searxng-<arch>".
* For example, a `container.yml` workflow is executed after being chained from `integration.yml` (called "~1"), and seconds later it may be triggered again because another PR merged some breaking changes (called "~2"). While "~1" has already passed the test job successfully and is about to start the release job, "~2" finishes building the container and overwrites the references on the common tag. When "~1" in the release job loads the images using the common tag, it will load the container of "~2" instead of "~1" having skipped the whole test job process.
The example is only set for the container workflow, but the other workflows might occur in a similar way.
Ivan Gabaldon