Only set new GitHub SSH key and PAC if needed.

[a-t-0]

• 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: Allow setting GitHub commit status using ssh key. #81)
• First check if such a GitHub ssh key exists locally, and if yes:
• Verify if it works, if it does work, skip adding a GitHub ssh key.
• Then check if such a GitHub personal access token exists locally, and if yes:
• 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.