Fix GitHub Actions for patches

[ESKYoung]

The govcookiecutter template build GitHub Action tries to mimic how a user would build the template. It uses GitHub environment variables to define the GitHub repository that cookiecutter should access.

However, if a PR is not raised from within ukgovdatascience, which will be the case for external contributors who raise patches, this Action fails, as the GITHUB_REPOSITORY variable refers to ukgovdatascience/govcookiecutter not the fork itself. This behaviour is seen in PR #39.

Also, the govcookiecutter build Action does not run for patches, so this needs to be triggered for PRs as well!