Increase rate limits for GitHub API requests
Increase rate limits for GitHub API requests

We're right on the cusp of breaching the standard 60 API hits
that are allowed for unauthenticated requests to the GitHub API.
GitHub Actions automatically create a GITHUB_TOKEN for authenticated
requests: https://docs.github.com/en/free-pro-team@latest/actions/reference/authentication-in-a-workflow#using-the-github_token-in-a-workflow

We just need to expose that as an ENV variable to each build task.
Using a token increases our limit to 15000 requests per hour.

Before:

```
2020-10-28T08:24:09.4012886Z x-ratelimit-limit: "60"
2020-10-28T08:24:09.4013490Z x-ratelimit-remaining: "1"
2020-10-28T08:24:09.4014106Z x-ratelimit-reset: "1603877018"
2020-10-28T08:24:09.4014667Z x-ratelimit-used: "59"
```

After:

```
x-ratelimit-limit: "15000"
x-ratelimit-remaining: "14995"
x-ratelimit-reset: "1603879429"
x-ratelimit-used: "5"
```

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs