CARSON CITY, Nev. — Retired Army Capt. Sam Brown, who lost Nevada’s 2022 GOP Senate primary, filed his formal candidacy Thursday for the seat held by first-term Sen. Jacky Rosen in a race Republicans have targeted nationally as one of their best chances to knock off an incumbent Democrat.Brown, a Purple Heart recipient, has been considered the early GOP front-runner in a crowded primary field since he announced he was running last summer, less than a year after he lost his bid to…