Tests: Trigger a fresh build of Boost Union Child in Github Actions when a new Boost Union Release is published

[abias]

Currently, Boost Union Child copies all Behat Scenarios from Boost Union and runs them, see https://github.com/moodle-an-hochschulen/moodle-theme_boost_union_child/blob/main/.github/workflows/moodle-plugin-ci.yml#L70-L81.

It would be nice to trigger a fresh build of Boost Union Child in Github Actions when a new Boost Union Release is published to be sure that newly added features or changes of Boost Union do not break in Boost Union Child.