Auto-cancel pipelines only on MRs

The interruptible keyword determines whether pipelines can be auto-cancelled.

For kernel testing pipelines, this should only be done for MRs. Baseline pipelines on ark-latest shouldn't cancel but run to completion.

For all other non-MR pipelines (scheduled/HEAD os-build), these should also not be cancelled.

Therefore, use a conditional include to only configure jobs to be interruptible in MR pipelines.

Fixes #119 (closed)