Replace CI test link for Arch Linux with external instance

- name: Arch Linux

  url: https://www.archlinux.org/

  logo: archlinux.png

  tests: https://tests.reproducible-builds.org/archlinux/

  tests: https://reproducible.archlinux.org/

  resources:

  - name: Developer wiki page

    url: https://wiki.archlinux.org/index.php/DeveloperWiki:ReproducibleBuilds

    url: /docs/recording/#arch-linux

  - name: verification tool for users

    url: https://github.com/archlinux/archlinux-repro

  - name: Rebuilds by Arch Linux

    url: https://reproducible.archlinux.org/

- name: Baserock

  url: https://baserock.org/

  logo: baserock.png