github: Pin a specific image for QEMU for Docker

This fixes temporary breakage in building arm64 docker images on
x86_64; see https://github.com/docker/setup-qemu-action/issues/198
and https://github.com/tonistiigi/binfmt/issues/240 for details
about this issue.

.github/workflows/docker.yml

@@ -93,6 +93,8 @@ jobs:
       - name: Set up QEMU
         if: ${{matrix.file == 'Dockerfile.toolchain'}}
         uses: docker/setup-qemu-action@v3
+        with:
+          image: tonistiigi/binfmt:qemu-v7.0.0-28
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
       - name: Log in to Docker Hub