Move QEMU action to after build step

It should not be required for building, only for testing

Signed-off-by: Kyle Harding <kyle@balena.io>

.github/workflows/docker.yml

@@ -37,8 +37,6 @@ jobs:
             type=ref,event=pr
             type=sha
 
-      - uses: docker/setup-qemu-action@v1
-
       - uses: docker/setup-buildx-action@v1
         with:
           driver-opts: network=host
@@ -56,6 +54,8 @@ jobs:
             ghcr.io/${{ env.slug }}:latest
           push: true
 
+      - uses: docker/setup-qemu-action@v1
+
       - name: Test functionality
         run: |
           for platform in $(tr ',' '\n' <<< "${{ env.platforms }}") ; do