nci src jobs add bionic repo

[jriddell]

The nci src jobs add bionic as a source repo even on xenial builds
This breaks on release-lts which has no bionic build.

in ci-tooling/nci/lib/setup_repo.rb add_source_repo! does a loop over NCI.series.each_key do |dist| which is why it adds both

this isn't the case with add_repo! which just calls debline() which adds LSB::DISTRIB_CODENAME

I'm not sure why they decide which release versions to add is different

[jriddell]

read the code. this is intentional
lts failing is obviously a problem though
probably simply need the empty repos published

[jriddell]

above comments from sitter