gitlab-ci: Mark jobs as interruptible

Allowing redundant pipelines to be automatically cancelled.
3 jobs for gitlab-ci-interruptible in 5 minutes and 36 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #1227731
debian-testing

00:00:40

 
  Build And Test
passed #1227732
autotools-build-and-test

00:04:56

passed #1227733
meson-build-and-test

00:01:55