Packer: Use CentOS redirect service for ISO images

We shouldn't be using CentOS ISO mirror of our choosing but their ISO
redirect service to get the image.

.gitlab-ci.yml

@@ -6,7 +6,7 @@ variables:
   DOCKER_DRIVER: overlay2
   DOCKER_TLS_CERTDIR: ""
   CI_REGISTRY_IMAGE: registry.gitlab.isc.org/isc-projects/images
-  CENTOS_ISO_MIRROR_URL: http://mirror.oss.ou.edu/centos
+  CENTOS_ISO_MIRROR_URL: http://isoredirect.centos.org/centos
   KYUA_GIT_COMMIT: 2fc3c21c30f9c52efaacb73f6f8b8dc716cf4bc2
 
 stages: