CI: build manifest for all update channels

Build the manifest for stable, beta and nightly in the package step.
This avoids the need of a rebuild to publish the firmware on a
different channel.

.gitlab-ci.yml

@@ -132,9 +132,8 @@ target:x86-64:
 package:
   stage: package
   script:
-    - GLUON_BRANCH=$(./get-gluon-branch.sh)
-    - test -z "$GLUON_BRANCH" || make -C gluon update GLUON_SITEDIR="$CI_PROJECT_DIR"
-    - test -z "$GLUON_BRANCH" || make -C gluon manifest GLUON_BRANCH=$(./get-gluon-branch.sh) GLUON_SITEDIR="$CI_PROJECT_DIR" V=1
+    - make -C gluon update GLUON_SITEDIR="$CI_PROJECT_DIR"
+    - for branch in "stable" "beta" "nightly"; do make -C gluon manifest GLUON_BRANCH="$branch" GLUON_SITEDIR="$CI_PROJECT_DIR" V=1; done
   artifacts:
     paths:
       - gluon/output