It’s safe to say today’s mainstream country musicians wouldn’t be who they are today without the country music that ruled the airwaves in the ‘90s. CMT’s Hot 20 Countdown will highlight that connection with an all-new concert special premiering Aug. 4-5 at 9 a.m. ET. Brothers Osborne, Easton Corbin, Carly Pearce, Lucie Silvas and hit-maker…