Setup a SSH deploy key so that workflow steps can fetch dependencies from private GitHub repos

my-workflow.yml

# .github/workflows/my-workflow.yml
# ... other config here
jobs:
    build:
        runs-on: ubuntu-18.04
        steps:
            -   uses: actions/checkout@v1
            -   name: Setup SSH Keys and known_hosts
                env:
                    SSH_AUTH_SOCK: /tmp/ssh_agent.sock
                run: |
                    mkdir -p ~/.ssh
                    ssh-keyscan github.com >> ~/.ssh/known_hosts
                    ssh-agent -a $SSH_AUTH_SOCK > /dev/null
                    ssh-add - <<< "${{ secrets.SSH_PRIVATE_KEY }}"
            -   name: Some task that fetches dependencies
                env:
                    SSH_AUTH_SOCK: /tmp/ssh_agent.sock
                run: ./fetch-deps.sh