Use environment variable for github/gitlab/... credentials

[guibou]

Right now the setting of credentials is done using cli argument.

We can improve that by using environment variables.

I propose to use KRANK_GITHUB_TOKEN=token, as well as KRANK_GITLAB_TOKENS=gitabhost=token;gitlabhost2=token2

cc @zimbatm who suggested the idea.

[zimbatm]

Fallback on GITHUB_TOKEN if it's set.

[sternenseemann]

It would be even better if those environment variables would point to credential files (like e.g. agenix uses, systemd supports via LoadCredential=) since those can't leak beyond their file permissions. The tokens in env vars can leak into subprocesses and tokens in arguments leak via /proc etc.