Pull #12942: Fix concurrency group for site action

[stoyanK7]

Resolves #12853 (comment)

The workflow uses this group
group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}

But this workflow context does not have github.event.pull_request.number, rather github.event.issue.number so it falls back to github.ref.

Print of new concurrency group:
https://github.com/stoyanK7/checkstyle/actions/runs/4578224927/jobs/8084568564#step:4:8

Site-62

Old one was Site-refs/heads/master.

[romani]

Ok to merge