Remove all uses of BUILD_NUMBER from the runner

[stronk7]

This is, simply about to remove all the uses of BUILD_NUMBER from the repository.

The affected places are marked in code with:

# TODO: Remove this once https://github.com/moodlehq/moodle-local_ci/issues/303 is fixed

Of course this only can be done once that #303 issue is closed.

Ciao :-)