don't spam duplicate warnings in github_context

[chris-laplante]

Thanks to https://github.com/go-git/go-git/issues/530, act will spam the console/log with a billion warnings like this:

WARN[0000] unable to get git repo: branch config: invalid merge

when a GitHub PR branch is checked out.

[mergify[bot]]

@chris-laplante this pull request is now in conflict 😩

[github-actions[bot]]

PR is stale and will be closed in 14 days unless there is new activity