CI: disable auto-cancel for master branch

[fangchenli]

#41962 enabled auto-cancel for GitHub Actions on both master and PRs. Ideally, we should only have this feature turned on for PRs.

Auto-cancel is achieved with the concurrency feature. It looks like this:

    concurrency:
      group: ${{ github.ref }}-xxx
      cancel-in-progress: true

The group attribute specifies a group name. So only one job can run for each group.

To disable it for master, we need to select the appropriate workflow/job variables from https://docs.github.com/en/actions/reference/context-and-expression-syntax-for-github-actions for the group name so that it's different for each commit to the master but identical for each PR.

[fangchenli]

Builds on master are been canceled recently. Not sure why.

[lithomas1]

Yep, this didn't work. Looking into it rn.

[lithomas1]

I think the problem here is that when three or more commits to master are merged, the first one runs, the second one queues up fine waiting for the first one to finish, but the third commit cancels the second commits run and queues up itself.

[lithomas1]

There may be hope. Gonna try https://github.community/t/concurrecy-not-work-for-push/183068/7 out.