Power supply units for mining rigs made by Bitmain Technologies Ltd.Photographer: BloombergA Chinese manufacturer that sells most of the world’s Bitcoin-mining machines — including 16,000 of them to a venture backed by two of President Donald Trump’s sons — has been the focus of a months-long federal investigation to assess whether its products pose risks to US national security.The use of Beijing-based Bitmain Technologies Ltd.’s machines at a site near a military base raised “significant natio...