BRUSSELS — Ukrainian steelmakers say the EU's new carbon border tax is prompting European customers to cancel orders, threatening one of the war-torn country's most important sources of export revenue. The Carbon Border Adjustment Mechanism, which came into force in January, requires importers to pay a carbon fee on certain foreign-made industrial goods, including steel. The point is to wean the dirtiest industries off planet-warming fossil fuels. But Ukrainian steel giants, struggling with the