Use SSH_KEY for ssh

[ViralBShah]

We had to do this in a couple of places for this to work reliably.

Example: JuliaInterop/RCall.jl#467

[DilumAluthge]

It just depends on what you choose to name the secret in your repository settings. Many users already have a secret named DOCUMENTER_KEY in their repo (for use by Documenter.jl), so we chose that as a default. If your secret is named something different, you'll need to change the name in the workflow file to match the name of the secret. So there isn't one workflow file that will work for everyone out of the box. But I think DOCUMENTER_KEY is a decent default.

[DilumAluthge]

Instead of changing the name of the default, can you add a comment line explaining that users need to change this value to match whatever they chose to name their secret?

[ViralBShah]

This is going beyond my understanding of github infra! Perhaps someone can update my PR or make a new one?