Restore docker/setup-qemu-action

Actually, we need this because LLVM doesn't cross-compile its
own build-time tools. We'll likely need this for Halide, too,
for similar reasons.

.github/workflows/build.yml

@@ -41,6 +41,11 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3.2.0
+        with:
+          platforms: "arm64"
+
       - name: Build and push image
         uses: docker/build-push-action@v6.7.0
         with: