changelog-github - infer repo name from environment variables #1870

Currently, when using @changesets/changelog-github, users have to always define the repo name as options: ["@changesets/changelog-github", { "repo": "org/repo" }]

However, in GitHub Actions, the repo name is also available as an environment variable GITHUB_REPOSITORY (see https://docs.github.com/en/actions/reference/workflows-and-actions/variables).

It would simplify usage if the changelog function used that as a fallback, and only throw error if neither of them (env var or options.repo) are defined.

When running locally, this changelog generator already requires user to set up .env with GITHUB_TOKEN and some other env vars, so doesn't make it more complex IMO - also, I think this is mostly used in CI anyway.

Happy to send a PR implementing this.