fix(deps): update dependency buildx to v0.21.1

| datasource      | package       | from    | to      |
| --------------- | ------------- | ------- | ------- |
| github-releases | docker/buildx | v0.20.1 | v0.21.1 |

Author: renovate[bot]

linux/Dockerfile

@@ -44,7 +44,7 @@ RUN install-tool flux v2.4.0
 RUN install-tool kubectl v1.32.2
 
 # renovate: datasource=github-releases packageName=docker/buildx
-RUN install-tool buildx v0.20.1
+RUN install-tool buildx v0.21.1
 
 # renovate: datasource=github-releases packageName=kubernetes-sigs/kustomize
 RUN install-tool kustomize 5.6.0