gitlab-ci: attempt at more reliably issuebot triggering on MRs

The only/except rules in all the jobs are quite complicated between
lint, fdroid build, issuebot, and checkupdates.  It seems that issuebot
never gets triggered on most merge requests, with most of the time, the
job isn't even started.  The special 'merge_requests' rule comes with all
sorts of complicated caveats, so hopefully this simpler setup works.

.gitlab-ci.yml

@@ -61,10 +61,12 @@ lint:
 
 trigger-issuebot:
   image: alpine
-  only:
-    - branches
   except:
     - master@fdroid/fdroiddata
+    - pipelines
+    - schedules
+    - tags
+    - triggers
   variables:
     GIT_DEPTH: "1"
   script: