allow_failure of mailbox test to make it optional again

.gitlab-ci.yml

@@ -104,10 +104,9 @@ mailbox integration test:
   rules:
     - if: '$CI_PIPELINE_SOURCE == "schedule"'
       when: on_success
-      allow_failure: false
     - if: '$CI_COMMIT_TAG == null'
       when: manual
-      allow_failure: false
+      allow_failure: true # TODO figure out how not to allow failure while leaving this optional
   script:
     # start mailbox
     - cd /opt && git clone --depth 1 https://code.briarproject.org/briar/briar-mailbox.git briar-mailbox