Fix the null ${{ github.event.pull_request.number }} in concurrency group

[fengyuentau]

Fixes opencv/opencv#22720

Notes:

• This bug is introduced in Cancel old jobs for the same PR if new changes happened #74
  • The problem is ${{ github.event.pull_request.number }} is null in concurrency, leading to jobs from different pull requests being put in a same lineup.
• Github documentation suggests using group: ${{ github.workflow }}-${{ github.ref }}.
• Github docs for context like ${{ github.xx }}:
  • https://docs.github.com/en/actions/learn-github-actions/contexts#needs-context
  • https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows.

See https://github.com/opencv/ci-gha-workflow/actions/runs/3469664748 and the name has no empty pull request number any more.

[alalek]

Documentation link for github.ref: https://docs.github.com/en/actions/learn-github-actions/contexts#github-context

The ref given is fully-formed, meaning that for branches the format is refs/heads/<branch_name>, for pull requests it is refs/pull/<pr_number>/merge, and for tags it is refs/tags/<tag_name>. For example, refs/heads/feature-branch-1.

[fengyuentau]

@asmorkalov Could you take a look at this PR and merge ASAP? It should solve the conflicts of contributors' commits cancelling each other.