adjust Citus package/version selection

Dockerfile

@@ -94,8 +94,11 @@ SHELL [ "/bin/sh", "-ec" ]
 
 COPY /apt/sources.citus  /etc/apt/sources.list.d/citus.sources
 
-RUN apt-install.sh \
-      "postgresql-${PG_MAJOR}-citus-12.1" \
+RUN apt-env.sh apt-get update ; \
+    citus_pkg=$(apt-cache search "^postgresql-${PG_MAJOR}-citus-[0-9.]+\$" | awk '{print $1}' | sort -rV | head -n1) ; \
+    : "${citus_pkg:?}" ; \
+    apt-install.sh \
+      "${citus_pkg}" \
       "postgresql-${PG_MAJOR}-topn" \
     ; apt-clean.sh ; \
     jdupes -1LSpr /usr/