Removed in e663d1dd

Use e3-build and a specification, to be able to select also which
versions of the EPICS modules which will be used. We will now also
install e3-base as part of this job.

We have to use different variables for the e3-base download compared to
the specification substitution, since the archive on artifactory has
had its filename modified to be linux compatible (no slash).

This commit also updates the version of e3-base used.

Set EPICS base version in `.gitlab-ci.yml` instead of project settings,
to make it more transparent as well as to make it work also for forks.