Use SSH key from input instead of file

[jswny]

It is possible to store the SSH key in secrets storage. This should be done so that an encrypted version doesn't need to be committed to the repository.

• https://www.webfactory.de/blog/use-ssh-key-for-private-repositories-in-github-actions
• https://github.community/t/deploy-to-remote-server-using-ssh-fails-on-host-key-verification-failed/17734/7