Work around MySQL server upgrade for CI

The latest Ubuntu 20.04 image from GitHub Actions comes with MySQL
server preinstalled in a custom way, so it can't be upgraded directly.

.github/workflows/build_and_test.yml

@@ -53,7 +53,13 @@ jobs:
       - name: Log perl information
         run: perl -V
       - name: Install packages
-        run: sudo apt-get install -y libmagic-dev default-mysql-server rsync aspell
+        run: sudo apt-get install -y libmagic-dev rsync aspell
+      - name: Work around MySQL server upgrade
+        run: |
+          wget -c https://dev.mysql.com/get/mysql-apt-config_0.8.20-1_all.deb
+          sudo dpkg -i mysql-apt-config_0.8.20-1_all.deb
+          sudo apt-get update
+          sudo apt-get install -y mysql-server
       - name: Install build dependencies
         run: cpanm --local-lib ${GITHUB_WORKSPACE}/local Dist::Zilla Perl::Critic Perl::Tidy Devel::Cover::Report::Coveralls Pod::Coverage::CountParents
       - name: Configure Dist::Zilla