Revert "Bump docker/build-push-action from 5 to 6"

This reverts commit 0a9d9ef.

Seems like actions/download-artifact got broken around the same time as
this change, so let's revert this change for now.

.github/workflows/build.yml

@@ -60,7 +60,7 @@ jobs:
         id: meta
         with:
           images: ghcr.io/${{ github.repository_owner }}/ruby.wasm/builder/${{ matrix.entry.target }}
-      - uses: docker/build-push-action@v6
+      - uses: docker/build-push-action@v5
         with:
           context: builders/${{ matrix.entry.target }}
           push: ${{ github.ref == 'refs/heads/main' || github.ref == 'refs/tags/*' }}
@@ -113,7 +113,7 @@ jobs:
           path: ./rubies
           key: ${{ matrix.entry.rubies_cache_key }}
       - uses: docker/setup-buildx-action@v3
-      - uses: docker/build-push-action@v6
+      - uses: docker/build-push-action@v5
         id: builder-image
         env:
           DOCKER_BUILD_NO_SUMMARY: true