CI build jobs dont run on forks without access to a secure runner

.gitlab-ci.yml requires that the master branch, schedules and tags are built on a secure runner. This works for archlinux/archiso>, but forks may not have access to secure runners.

From reading https://gitlab.archlinux.org/help/ci/yaml/README.md#onlyexcept-basic , it should be possible to limit this to a specific repository:

build:
  extends: .build
  tags:
    - fast-single-thread
  except:
    - master@archlinux/archiso
    - schedules@archlinux/archiso
    - tags@archlinux/archiso

build:secure:
  extends: .build
  tags:
    - secure
  only:
    - master@archlinux/archiso
    - schedules@archlinux/archiso
    - tags@archlinux/archiso

But, it's unclear to me if @archlinux/archiso only works for branches or for schedules and tags too. @svenstaro?