ci: Add workflow:auto_cancel flag to pipeline settings

GitLab now supports the flag workflow:auto_cancel to automatically cancel pipelines when new commits are pushed to the branch. This change helps reducing the consumption of pipeline minutes and resources on outdated commits, which is frequent when editing MRs.

This MR proposes to add that setting to cancel pipelines that were superseded by newer commits. If the pipeline is running on the master branch, the pipeline is not cancelled. Running full pipelines on the master branch helps tracking down possible regressions introduced by newer commits.

Moreover, set all jobs as interruptible, except manual jobs and the release stage.