Mark backports CI job as non-interruptible

Previously CI job for the autobackport bot inherited "interruptible:
true" global configuration. This caused premature termination of the job
when another merge was finished before the autobackport job ran to
completion.

(cherry picked from commit 3165261ecd)

.gitlab-ci.yml

@@ -1957,6 +1957,8 @@ pairwise:
       - $PAIRWISE_TESTING
 
 backports:
+  # finish backport attempt even if another MR was merged while the job was running (or queued)
+  interruptible: false
   <<: *base_image
   stage: postmerge
   rules: