Updating cirrus docker image to ubuntu focal. (flutter#128291)

Bug:https://bugs.chromium.org/p/chromium/issues/detail?id=1446086

Author: yusuf-goog

dev/ci/docker_linux/Dockerfile

@@ -12,9 +12,12 @@
 
 # Last manual update 2021-09-24 (changing this comment will re-build image)
 
-FROM ubuntu:bionic@sha256:eb1392bbdde63147bc2b4ff1a4053dcfe6d15e4dfd3cce29e9b9f52a4f88bc74
+FROM ubuntu:focal@sha256:b795f8e0caaaacad9859a9a38fe1c78154f8301fdaf0872eaf1520d66d9c0b98
 MAINTAINER Flutter Developers <[email protected]>
 
+ENV TZ=America/Los_Angeles
+RUN ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone
+
 RUN apt-get update -y && \
   apt-get upgrade -y