LONDON (Reuters) -Rishi Sunak, trailing in the race to become Britain’s next prime minister, has vowed to slash the basic rate of income tax by 20% by 2029 in a potentially make-or-break throw of the dice by the former finance minister. Sunak, once seen as the favourite to replace Boris Johnson when he helped to steer the economy through the ravages of the COVID-19 pandemic, has struggled against his rival, Foreign Secretary Liz Truss, who has pledged immediate tax cuts. Sunak said he remained focused on tackling inflation but once that was achieved he would follow through on an already-announ…