Update CI to run on merge requests

# CI Updates

This adds the rule: `if: $CI_PIPELINE_SOURCE == 'merge_request_event'` to allow the build pipeline to run on merge requests.

The GitLab docs state "The parent project’s `.gitlab-ci.yml` file must be configured to run jobs in merge request pipelines.", and this change intends to implement this requirement.

> Note, that since this is not already in the parent, this will not be a noticeable change for this specific MR, only MRs post the merge of this.