ci: bump docker/build-push-action to v6

v5 always fails, see e.g. https://github.com/bitcoin-core/HWI/actions/runs/16439512674/job/46456803381

Author: Sjors

.github/workflows/ci.yml

@@ -91,7 +91,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: docker/build-push-action@v5
+      - uses: docker/build-push-action@v6
         with:
           context: .
           file: contrib/build-wine.Dockerfile
@@ -115,7 +115,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: docker/build-push-action@v5
+      - uses: docker/build-push-action@v6
         with:
           context: .
           file: contrib/build.Dockerfile