fix(deps): update dependency buildx to v0.21.1 (#2122)

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

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