There’s some amazing news for fans of “The Amazing Race.”CBS has given exec producers Bertram van Munster and Jerry Bruckheimer the OK to begin casting for a fifth installment of the Emmy-nominated reality hit. While that’s not the same thing as an official series order, it’s a very good omen. Eye traditionally lets its reality producers begin casting only when there’s a solid shot of a pickup or renewal, particularly when it comes to existing franchises. “Amazing Race 4,” hosted by Phil Keoghan, was a solid Nielsen performer for CBS this summer in a difficult 8 p.m. Thursday slot, building ratings momentum as its season went along. Skein features couples in various relationships — friends, spouses, siblings — racing around the globe in a bid for $1 million.