validate_vars aborts if optional "github_token" not set

[mbarnes]

Was messing around with running papr locally and hit this right off the bat:

ERROR: Missing variable 'github_token'.

The github_token variable is supposed to be optional, according to RUNNING.md.

My workaround was just to disable the validate_vars call in main, or I guess remove github_token from the loop leaving only github_repo to validate.

[jlebon]

Yeah, that sounds about right. We regressed on this at some point. Though note the whole codebase is in flux while we move to Python, so some of the documentation might be off until things settle.