[Github] Fix concurrency groups for premerge

According to https://docs.github.com/en/rest/using-the-rest-api/github-event-types?apiVersion=2022-11-28,
When we look at the push event payload, github.event.push.head is a string
containing the SHA. This is currently causing new commits on main to cancel
the premerge pipeline of older commits.

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.sha }}
+      group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.event.push.head }}
       cancel-in-progress: true
     container:
       image: ghcr.io/llvm/ci-ubuntu-22.04:latest