[Github][CI] Remove premerge container (#123483)

This patch removes the container from the premerge job. We are moving
away from the kubernetes executor back to executing everything in the
same container due to reliability issues. This patch updates everything
in the premerge job to work.

This is part of a temp fix to
llvm/llvm-zorg#362.

.github/workflows/premerge.yaml

@@ -18,11 +18,6 @@ jobs:
     concurrency:
       group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
       cancel-in-progress: true
-    container:
-      image: ghcr.io/llvm/ci-ubuntu-22.04:latest
-    defaults:
-      run:
-        shell: bash
     steps:
       - name: Checkout LLVM
         uses: actions/checkout@v4