.github: add workflow_dispatch event
.github: add workflow_dispatch event

To allow manually running the workflow, as described in
https://docs.github.com/en/actions/managing-workflow-runs/manually-running-a-workflow

Signed-off-by: Kevin Locke <ffb4761cba839470133bee36aeb139f58d7dbaa9@kevinlocke.name>

kevinoid/travis-status,kevinoid/travis-status,kevinoid/travis-status