component/golang: clean build cache at the end of build

to save disk space

component/golang/buildout.cfg

@@ -208,6 +208,9 @@ command = bash -c ". ${gowork:env.sh}
 			;;
 		esac
 	done
+
+	# clean build cache
+	chmod -R a+rwx ${gowork:directory}/cache && rm -rf ${gowork:directory}/cache
 	"
 update-command = ${:command}
 stop-on-error = true