Support workflow_dispatch events on GitHub Actions

[mortenpi]

Currently Documenter won't deploy if you run a workflow with workflow_dispatch:

┌ Info: Deployment criteria for deploying devbranch build from GitHub Actions:
│ - :heavy_check_mark: ENV["GITHUB_REPOSITORY"]="..." occurs in repo="..."
│ - ✘ ENV["GITHUB_EVENT_NAME"]="workflow_dispatch" is "push"
│ - :heavy_check_mark: ENV["GITHUB_REF"] matches devbranch="master"
│ - :heavy_check_mark: ENV["GITHUB_ACTOR"] exists and is non-empty
│ - :heavy_check_mark: ENV["DOCUMENTER_KEY"] exists and is non-empty
└ Deploying: ✘

[richardreeve]

I just had the same issue with running my documentation workflow with release. It does seem like this should be easy to fix if it's really just here to stop pull requests triggering deployment.