CI: make build and test jobs interruptible

What does this MR do?

Build and test jobs are made interuptible.

When new commits were pushed to a branch/MR, previously pipelines still continued to run for older commits, potentially wasting resources. With the interruptible key, they will be automatically cancelled in favor of new commits' pipelines.

How confident are you it won't break things if deployed?

No application code touched

Links to related issues

https://yunity.slack.com/archives/C1T1ZM9A4/p1606276423338000

How to test

Create a new MR, and push two commits in quick succession. Verify that the pipeline on the older commit is automatically cancelled.

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.