[llvm] workflows/premerge: Cancel in progress jobs when a PR is merged (PR #125329)

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: 2deba08e09b9412c9f4e5888237e28173dee085b

https://github.com/llvm/llvm-project/actions/runs/13123075560

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/125329