By Fanny Brodersen, Anna Koper and Michael Kahn GORZOW WIELKOPOLSKI, Poland (Reuters) – Construction sites, factory assembly lines and warehouses across central Europe are scrambling to fill vacancies after tens of thousands of Ukrainian men left their blue-collar jobs to return home after Russia invaded their country. Ukrainian workers had flocked to central Europe in the past decade – drawn by higher wages and aided by an easing of visa requirements – filling jobs that weren’t highly paid enough for local workers in construction, the automotive sector, and heavy industry. Many of these worke…