build-jobs.jinja2: Specify master branch variable again

If the MASTER_BRANCH_MANIFEST variable is set in the manifest's
.gitlab-ci.yml file, it may not find its way to the generated job,
if it is not explicitly set during generation.
That is done with this patch.

build-jobs.jinja2

@@ -17,6 +17,9 @@ include:
     ref: {{ gitlab_ci_current_rev }}
     file: 'manifest-package.yml'
 
+variables:
+  MASTER_BRANCH_MANIFEST: {{ master_branch_manifest }}
+
 workflow:
   rules:
     # This rule is needed, as otherwise the workflow:rules from