gitlab-ci: Run configured linters as done on Phabricator
Provide a backward-compatible way to run linters for projects that have been configured for use with Phabricator/Differential.
Even without Phabricator,
arc lint is a good configurable frontend to run
per-language linters, so it's not a bad choice even without backward
This is the task that prompted this MR: https://phabricator.apertis.org/T6299
(!10 (merged) is an older version of this MR that was landed by mistake to v2020pre and has been reverted)