Fix CI tests with MariaDB. (#1797)

* Fix CI tests with MariaDB.

The docker image now uses MARIADB_ROOT_PASSWORD for the environment
variable for the password rather than MYSQL_ROOT_PASSWORD.

* It's mariadb-admin now not mysqladmin

As per MariaDB/mariadb-docker#497

.github/workflows/test.yml

@@ -20,9 +20,9 @@ jobs:
       mariadb:
         image: mariadb
         env:
-          MYSQL_ROOT_PASSWORD: debug_toolbar
+          MARIADB_ROOT_PASSWORD: debug_toolbar
         options: >-
-          --health-cmd "mysqladmin ping"
+          --health-cmd "mariadb-admin ping"
           --health-interval 10s
           --health-timeout 5s
           --health-retries 5