Only setup QEMU for ppc64le builds on GHA (#1000)

Author: mayeut

.github/workflows/build.yml

@@ -43,7 +43,10 @@ jobs:
         uses: actions/checkout@v2
 
       - name: Set up QEMU
+        if: matrix.platform == 'ppc64le'
         uses: docker/[email protected]
+        with:
+          platforms: ppc64le
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1