make explicit in the start_slots page which version of LHcbNightlyConf is used

When triggering a new slot build we may not use the latest version of LHCbNightlyConf (e.g. if the CI job on the master branch fail for whichever reason).

Which version is used is not clear in the page, so it may be confusing that if there's a problem. Making it more explicit would help debugging.