[polari] ci: Set a cache key

[Florian Müllner <fmuellner src gnome org>]

commit 53b9feabccc852c4e3a6308384a07a84027b5b44
Author: Florian Müllner <fmuellner gnome org>
Date:   Wed Jul 25 19:42:11 2018 +0200

    ci: Set a cache key
    
    Somehow the flatpak job keeps building all the dependencies more
    often than not (despite all the modules being tagged to a specific
    commit). I suspect gitlab throws the cache away when it encounters
    a job with a different (in this case: no) path; if that is the
    case, then specifying a key to use a non-default cache should
    fix it.

 .gitlab-ci.yml | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 738c500..e0768b3 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -41,6 +41,7 @@ flatpak:
             - _build/meson-logs/testlog.txt
         expire_in: 30 days
     cache:
+        key: flatpak-builder
         paths:
              - cache.tar.gz