Fail deploying documents since switching to GitHub Actions

[hyrodium]

There are no deployed documents on gh-pages branch since the PR #752.
https://github.com/JuliaArrays/StaticArrays.jl/tree/gh-pages

I think, this is because Documenter.auto_detect_deploy_system() is failing to detect GitHub Actions, and the function is detecting Travis mistakenly.

Here's a screenshot from GitHub Actions.

It should be like this:

The functions auto_detect_deploy_system() is detecting the system from environment variables, so we should check them.
https://github.com/JuliaDocs/Documenter.jl/blob/master/src/deployconfig.jl#L825

[fredrikekre]

Very old Documenter.jl version https://github.com/JuliaArrays/StaticArrays.jl/blob/master/docs/Project.toml#L5

[hyrodium]

Okay, I'll make a PR.