Install php-excimer

[aspark21]

Adding https://github.com/catalyst/moodle-tool_excimer to our build and it requires this extension, running our unit testing pipelines through https://github.com/moodlehq/moodle-docker so seems fitting to suggest to add here.
Looks to be a pretty nice tool to monitor Moodle, Cat AU / Brendan & team have implemented.

My quick attempt at adding has failed - aspark21@7e3d41e - https://github.com/aspark21/moodle-php-apache/runs/6594297672?check_suite_focus=true

Will keep digging tomorrow and see if I can get a working pull request for it