Documenter.jl: `push_preview` does not work in Github Actions

[asinghvi17]

With deploydocs(..., push_preview = true), I get this output in the log:

┌ Info: Deployment criteria for deploying devbranch build from GitHub Actions:
│ - ✔ ENV["GITHUB_REPOSITORY"]="JuliaPlots/MakieGallery.jl" occurs in repo="github.com/JuliaPlots/MakieGallery.jl"
│ - ✔ ENV["GITHUB_EVENT_NAME"]="push" is "push"
│ - ✘ ENV["GITHUB_REF"] matches devbranch="master"
│ - ✔ ENV["GITHUB_ACTOR"] exists
│ - ✔ ENV["DOCUMENTER_KEY"] exists exists
└ Deploying: ✘

https://github.com/JuliaPlots/MakieGallery.jl/pull/187/checks?check_run_id=411479592

The make.jl file is here: https://github.com/JuliaPlots/MakieGallery.jl/blob/master/docs/make.jl

[mortenpi]

The issue is that your documentation build runs on: push not on on: pull_request. Hence the GITHUB_EVENT_NAME is set to probably push, whereas Documenter expects pull_request.

For the PR previews to work, you need to make sure that your builds run as part of the PR (i.e. on: pull_request).