Inconsistencies with `buf_token`

[jchadwick-buf]

This was reported in the Slack channel.

1. The documentation claims the default value for buf_token is ${{github.token}}. This seems incorrect.
2. It seems that there is some inconsistency between some of the GitHub Actions. buf-setup-action uses the name buf_api_token whereas this and some other actions use buf_token. We should probably unify these at some point?