ci: Don't allow the nightly pipeline to be interrupted.

Since 58cfcc65 the default for jobs has been "interruptible", this means that when new commits are pushed to a branch which already has a running pipeline then the old pipelines for this branch are cancelled.

This includes the master branch, and in particular, new commits merged to the master branch will cancel the nightly job.

The semantics of pipeline cancelling are actually a bit more complicated though. The interruptible flag is per job, but once a pipeline has run any non-interruptible job, then the whole pipeline is considered non-interruptible (ref https://gitlab.com/gitlab-org/gitlab/-/issues/32837). This leads to the hack in this MR where by default all jobs are interruptible: True, but for pipelines we definitely want to run, there is a dummy job which happens first, which is interreuptible: False. This has the effect of dirtying the whole pipeline and

For now, this patch solves the immediate problem of making sure nightly jobs are not cancelled. In the future, we may want to enable this job also for the master branch, making that change might mean we need more CI capacity than currently available.

[skip ci]

Ticket: #19554 (closed)