ci: add CentOS 8 support; drop CentOS 7

Refs: #5040, #5087
Change-Id: Ia680fecc41d16d21d7b03f117a30a2966cf46679
diff --git a/.jenkins b/.jenkins
index 25bf743..287bdd7 100755
--- a/.jenkins
+++ b/.jenkins
@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 set -e
+source .jenkins.d/util.sh
 
+export CACHE_DIR=${CACHE_DIR:-/tmp}
 export WAF_JOBS=${WAF_JOBS:-1}
 
 nanos() {