update rocky8-eb-base: use the release version of 8.4

rocky8-eb-base/Dockerfile

 ARG IMG_REGISTRY=docker.io
 ARG IMG_ORG=rockylinux
 ARG IMG_NAME=rockylinux
-ARG IMG_TAG=8.4-rc1
+ARG IMG_TAG=8.4
 # allow CI/CD to change all of the above
 FROM ${IMG_REGISTRY}/${IMG_ORG}/${IMG_NAME}:${IMG_TAG}
 # set default easybuild version