Add workflow_dispatch to gh actions merge-checks.yml

[PlasmaPower]

This doesn't do anything on its own, but it's necessary to merge this before #2468 can be fully tested.

Source: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch

Note: This event will only trigger a workflow run if the workflow file is on the default branch.

[joshuacolvin0]

LGTM