gitlab-ci: temporary allow distcheck job failure.

I really don't like to flag the distcheck job as allowed to fail, but
the issue we have with it right now (#5790) is very annoying and I have
no idea where the weird uncleaned files come from. I can't reproduce
this locally and these files are seemingly never created here during a
distcheck.
Since it makes all our pipelines fail, this makes it harder to diagnose
and find real other bugs, so let's allow failure until we figure this
out.

.gitlab-ci.yml

@@ -143,6 +143,8 @@ gimp-autotools-debian:
     - make check
 
 gimp-distcheck-debian:
+  # Temporarily allow distcheck failure. See #5790
+  allow_failure: true
   extends: .gimp-debian-base
   script:
     - mkdir _build