CI: disable auto-cancel for master branch
#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:
```yaml
    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.
