Automatically cancel CI jobs on outdated branches

Gitlab feature
https://docs.gitlab.com/ee/ci/pipelines/settings.html#auto-cancel-redundant-pipelines
can automatically cancel jobs which operate on an outdated code, i.e. on
branches which received new commits while jobs with an older set of
commits are still running. For this feature to work jobs have to be
configured with boolean interruptible: true.

I think practically all of our current CI jobs can be cancelled,
so the option is now on by default for all jobs.

.gitlab-ci.yml

@@ -47,6 +47,13 @@ variables:
   BIND_STRESS_TEST_OS: linux
   BIND_STRESS_TEST_ARCH: amd64
 
+# Allow all running CI jobs to be automatically canceled when a new
+# version of a branch is pushed.
+#
+# See: https://docs.gitlab.com/ee/ci/pipelines/settings.html#auto-cancel-redundant-pipelines
+default:
+  interruptible: true
+
 stages:
   - autoconf
   - precheck