Fix default branch in master

(Problem after a merge from stable/potassium to master)

.gitlab-ci.yml

@@ -6,7 +6,7 @@ stages:
 
 variables:
     CURRENT: $CI_COMMIT_REF_NAME
-    DEFAULT: "stable/potassium"
+    DEFAULT: "master"
     FRAMA_CI_OPT: "--override frama-c:$CI_COMMIT_REF_NAME,$CI_COMMIT_SHA"
 
 #avoid a nix error https://github.com/NixOS/nix/issues/2087