[geary/wip/flatpak-nightly-mainline] Publish nightly builds from mainline, not master

[Michael Gratton <mjog src gnome org>]

commit 6f2190f4c657c310249a4de3a810d350340171fe
Author: Michael Gratton <mike vee net>
Date:   Thu Sep 26 10:02:02 2019 +1000

    Publish nightly builds from mainline, not master

 .gitlab-ci.yml | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 7d666396..10ea6d8d 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -100,4 +100,7 @@ stop_review:
     extends: .stop_review
 
 nightly:
-  extends: '.publish_nightly'
\ No newline at end of file
+  extends: '.publish_nightly'
+  only:
+    refs:
+      - mainline