Don't require a GitHub access token if in "skip publish" mode

[timrogers]

The skip-publish option allows you to build your Gatsby site but not publish it, "effectively performing a test of the build process using the live configuration". This is useful for CI purposes - i.e. checking your site builds, but not actually deploying.

However, at the moment, you have to provide an access-token when in "skip publish" mode, even though this is only used to publish.

This updates the action - including new tests - so that you are not required to specify the access-token option if you've asked to skip publishing.

[timrogers]

@enriikke Thanks for this nice GitHub Action ❇️