Add govuk-guix to the list of alphagov_repos
Add govuk-guix to the list of alphagov_repos

This makes it easier to use the govuk-data script, as it'll be
available by default if you clone all the repositories in the
alphagov_repos file (as is documented in the GOV.UK Developer docs).

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet