[Github] Fix premerge concurrency cancellation

This should actually fix the problem as I validated that github.sha returns an
actual value by running a workflow in a test repo. I'm not sure why the
existing value doesn't work, but it returns nothing.

Author: boomanaiden154

.github/workflows/premerge.yaml

@@ -16,7 +16,7 @@ jobs:
     if: github.repository_owner == 'llvm'
     runs-on: llvm-premerge-linux-runners
     concurrency:
-      group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.event.push.head }}
+      group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
       cancel-in-progress: true
     container:
       image: ghcr.io/llvm/ci-ubuntu-22.04:latest