gitlab-ci: Rename RELEASE variable to RELEASE_JOB

This interfered with the autoconf variable of the same name, breaking
pre-release builds.

.gitlab-ci.yml

@@ -67,7 +67,7 @@ workflow:
     when: always
     expire_in: 1 year
   rules:
-    - if: '$RELEASE == "yes"'
+    - if: '$RELEASE_JOB == "yes"'
 
 ############################################################
 # Runner Tags