[CI] Only run one post commit job on a push to a pull request in sycl-devops-pr (#16079)

Kind of a really specific case, but if someone pushes to
`sycl-devops-pr` to run postcommit and then makes a PR from the same
branch, both the initial pull request event and every subsequent push to
the branch will cause two postcommits to run, one from the push to the
branch and one from the PR.

Use a unique key that will be the same in both cases so only one is run
and the other is cancelled.

Nobody really uses this branch besides CI devs so we don't need to
overengineer it so it never runs two in the first place IMO.

---------

Signed-off-by: Sarnie, Nick <[email protected]>

Author: sarnex

.github/workflows/sycl-post-commit.yml

@@ -20,8 +20,11 @@ on:
     - ./devops/actions/cached_checkout
 
 concurrency:
-  #  Cancel a currently running workflow from the same PR or commit hash.
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  # Cancel a currently running workflow from the same PR or commit hash.
+  # We need to use the user's branch name (which is in different variables
+  # for pull request events and push events) so that making a PR from a
+  # sycl-devops-pr branch doesn't cause two postcommit runs.
+  group: "${{ github.actor }}-${{ github.head_ref || github.ref_name }}"
   cancel-in-progress: true
 
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