Concurrency for GitLab runners has to be set explicitly

I found out there's a concurrent setting for self hosted GitLab runners that has a default value of 1, preventing you from having several runners run at the same time:

Limits how many jobs can run concurrently, across all registered runners. Each [[runners]] section can define its own limit, but this value sets a maximum for all of those values combined. For example, a value of 10 means no more than 10 jobs can run concurrently. 0 is forbidden. If you use this value, the runner process exits with a critical error.

Despite having my setup running for several years, with 3 available runners, I never realised that jobs within the same stage in my pipelines were not running in parallel. They do now, after raising the value of concurrent.

While I was at it, I also changed my pipeline configuration. The last stage is called vanity and runs things like black and pylint to check whether the code follows conventions. I made these jobs have allow_failure set to true, in order to make GitLab not consider pipeline runs failed if any of these jobs don't turn out green.