chore(deps): update docker/setup-buildx-action action to v4 (#22)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

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

@@ -356,7 +356,7 @@ runs:
     # ~20-min osxcross compile. Loaded as a local tag the run step below consumes.
     - name: Set up buildx (cross-dockerfile legs)
       if: steps.tjs-cache.outputs.cache-hit != 'true' && inputs.cross-dockerfile != ''
-      uses: docker/setup-buildx-action@v3
+      uses: docker/setup-buildx-action@v4
     - 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