chore(deps): update docker/build-push-action action to v7

Author: renovate[bot]

.github/actions/build-leg/action.yml

@@ -356,7 +356,7 @@ runs:
       uses: docker/setup-buildx-action@v3
     - name: Build the cross-toolchain image (GHA-cached, no registry)
       if: steps.tjs-cache.outputs.cache-hit != 'true' && inputs.cross-dockerfile != ''
-      uses: docker/build-push-action@v6
+      uses: docker/build-push-action@v7
       with:
         context: ${{ inputs.cross-dockerfile }}
         load: true