Only set new GitHub SSH key and PAC if needed.

[a-t-0]

• [x] To speed up the installation and dev/testing process, only create a GitHub SSH key and PAC if needed. Determine whether a function already exists that performs such a check. (Hint: https://github.com/TruCol/Self-host-GitLab-CI-for-GitHub/issues/81)
• [x] First check if such a GitHub ssh key exists locally, and if yes:
• [x] Verify if it works, if it does work, skip adding a GitHub ssh key.
• [x] Then check if such a GitHub personal access token exists locally, and if yes:
• [x] Verify if it works, if it does work, skip adding a GitHub personal access token.

Relevant code:

set_personal_github_pat_and_verify() 
# TODO: first verify if the GitHub pat exists and can be used, before
    # creating a new one.

[a-t-0]

Done, it works.