fill in default github environment variables

[frantjc]

the sequence cli should try to parse information from the .git and .github/ folders in the current directory to fill in the default environment variables that github actions supplies: https://docs.github.com/en/actions/learn-github-actions/environment-variables#default-environment-variables

such variables should be overridable on all sqnc run commands via flags

[frantjc]

looking like some of the environment variables are not going to be able to be parsed locally; we should be able to call out to GitHub's API to try and retrieve some

the primary problematic one will be GITHUB_TOKEN. ideally we can prompt the user for credentials and then call out to GitHub for it, but we should also allow the user to override it.

[frantjc]

@moeller142 this is the issue for what we were just talking about (getting GitHub env vars from CLI, remote, .git dir, etc

[frantjc]

pretty sure everything gets filled in from local git repositories now except head ref and base ref. need to be able to do the same from remote repositories for sequence to be able to scale