[CI] Use Ubuntu 20.04 Docker image for CI tests (#9007)

They don't seem to crash.

Supersedes #7775.

.ci/before_install.sh

@@ -1,17 +0,0 @@
-#!/usr/bin/env bash
-
-# don't do this for clang
-if [ "${CXX}" = "g++" ]; then
-    export CXX="g++-5" CC="gcc-5"
-fi
-# in case anything ignores the environment variables, override through PATH
-mkdir bin
-ln -s "$(command -v gcc-5)" bin/cc
-ln -s "$(command -v gcc-5)" bin/gcc
-ln -s "$(command -v c++)" bin/c++
-ln -s "$(command -v g++-5)" bin/g++
-
-# Travis only makes a shallow clone of --depth=50. KOReader is small enough that
-# we can just grab it all. This is necessary to generate the version number,
-# without which some tests will fail.
-# git fetch --unshallow

.circleci/config.yml

@@ -16,7 +16,7 @@ workflows:
 jobs:
   build:
     docker:
-      - image: koreader/koappimage:0.1.8
+      - image: koreader/kobase:0.3.0
         auth:
           username: $DOCKER_USERNAME
           password: $DOCKER_PASSWORD
@@ -37,9 +37,6 @@ jobs:
           keys:
             - build-{{ arch }}-{{ checksum "git-rev-base" }}
       - run: echo 'export PATH=${HOME}/bin:${PATH}' >> $BASH_ENV
-      - run:
-          name: setup
-          command: .ci/before_install.sh
 
       # installs and caches testing tools
       - run: