CI: migrate deprecated 'linux' tags
Gitlab.com is being upgraded to 17.0 on 2024-05-16,¹ a breaking change that
removes the linux
tag usability.²
¹ https://about.gitlab.com/blog/2024/04/10/a-guide-to-the-high-impact-breaking-changes-in-gitlab-17-0/
² https://about.gitlab.com/blog/2023/08/15/removing-tags-from-small-saas-runner-on-linux/