Makefile: update buildx version to v0.6.0
.PHONY: all binary dynbinary build cross help install manpages run shell test test-docker-py test-integration test-unit validate win
-BUILDX_VERSION ?= v0.5.1
+BUILDX_VERSION ?= v0.6.0
ifdef USE_BUILDX
BUILDX ?= $(shell command -v buildx)
