Revert to using Ubuntu 18.04 for the wrk image (#7832)

toolset/wrk/wrk.dockerfile

@@ -1,4 +1,4 @@
-FROM buildpack-deps:jammy
+FROM buildpack-deps:bionic
 
 RUN apt-get update && apt-get install -yqq libluajit-5.1-dev libssl-dev luajit