Use ssh key saved in ssh-agent if deploy key env var is not set. Fixes #398

[raquo]

@olafurpg Thanks for the pointer in #398, I think this should fix it.

• If neither env variable is found, we now set DEPLOY_KEY to empty string instead of throwing an error.
• Code proceeds to if [[ -n "${DEPLOY_KEY:-}" ]]; then. This checks that DEPLOY_KEY is set AND non-empty, so this check fails, and the code proceeds into else, where a warning is printed.
• Code then proceeds to run yarn install etc. If you have ssh-add-ed an ssh key like I do locally, this will use that key, and your name & email (not docusaurus info from set-up-ssh), which... I guess is appropriate given that your personal key is used?

This change works for me, I tested by running this script locally on bash 3.2.

I thought about updating the docs to indicate this new behaviour, but the way they read heavily implies this new behaviour already I think.

[olafurpg]

Thank you! Let’s give this a try 💪