[gnome-runtime-images] ci: rename the rebuild rule variable

[Jordan Petridis <jpetridis src gnome org>]

commit d1e4b6f840e8e081e9474a3d5426ff47c76e9be4
Author: Jordan Petridis <jordan centricular com>
Date:   Mon Oct 18 01:41:16 2021 +0300

    ci: rename the rebuild rule variable
    
    So it does not have to be updated with every release

 .gitlab-ci.yml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 2581683..bc6c005 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -117,7 +117,7 @@ flat-manager-client:
     BRANCH: "40"
     FD_BRANCH: "20.08"
   rules:
-    - if: '$REBUILD_40'
+    - if: '$REBUILD_STABLE'
       when: 'always'
     - when: 'manual'
       allow_failure: true
@@ -127,7 +127,7 @@ flat-manager-client:
     BRANCH: "41"
     FD_BRANCH: "21.08"
   rules:
-    - if: '$REBUILD_41'
+    - if: '$REBUILD_STABLE'
       when: 'always'
     - when: 'manual'
       allow_failure: true