ci/prow-rhcos.sh: fix branch handling
`BRANCH` is the cosa branch name, `RHCOS_BRANCH` is the mapped branch to
use for checking out the RHCOS config.
@@ -15,5 +15,5 @@ esac
export COSA_SKIP_OVERLAY=1
# Create a temporary cosa workdir
cd "$(mktemp -d)"
-cosa init -b "${BRANCH}" https://github.com/openshift/os
+cosa init -b "${RHCOS_BRANCH}" https://github.com/openshift/os
exec src/config/ci/prow-build-test-qemu.sh
