Add CI related env variables to legacy_tox_ini

It would be useful to add `passenv` option to the `legacy_tox_ini` section. Currently, you can't use environmental variables to check if you're running on CI from within Python if `tox` is used. I've been caught by this a couple of times now so I think it'd be good to populate it by default.

```
passenv =
    CI
    GITHUB_ACTIONS
```