[CI] Cancel postcommit jobs on new commit (#14606)

You can see it seems to work
[here](https://github.com/intel/llvm/actions/runs/9980873784).

Example:

```
build-win / Build + LIT
Canceling since a higher priority waiting request for 'SYCL Post Commit-14606' exists
```

---------

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

Author: sarnex

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

@@ -19,6 +19,11 @@ on:
     - ./devops/actions/cleanup
     - ./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-in-progress: true
+
 permissions: read-all
 
 jobs: