lister: Make sure lister that requires github tokens can use it

Deploying the nixguix lister, I realized that even though the credentials configuration
is properly set for all listers, the listers actually requiring github origin
canonicalization do not have access to the github credentials. It's lost during the
constructor to only focus on the lister's credentials. Which currently translates to
listers being rate-limited.

This commit fixes it by pushing the self.github_session instantiation in the constructor
when the lister explicitely requires the github session. Hence lifting the rate limit
for maven, packagist, nixguix, and github listers.

Related to infra/sysadm-environment#4655