fix manifest generation

When FFS_BUILD_DATE is not set when make manifest is executed, an manifest
without any images will be generated.

.gitlab-ci.yml

@@ -147,7 +147,7 @@ target:x86-64:
 package:
   stage: package
   script:
-    - for branch in "stable" "beta" "nightly" "experimental"; do make -C gluon manifest GLUON_BRANCH="$branch" GLUON_SITEDIR="$CI_PROJECT_DIR" V=1; done
+    - for branch in "stable" "beta" "nightly" "experimental"; do make -C gluon manifest GLUON_BRANCH="$branch" GLUON_SITEDIR="$CI_PROJECT_DIR" FFS_BUILD_DATE=$(cat build.date) V=1; done
     - if [ -n "$GLUON_SIGN_KEY" ]; then cd gluon && for branch in "nightly" "experimental"; do ./contrib/sign.sh <(echo "$GLUON_SIGN_KEY") "output/images/sysupgrade/$branch.manifest"; done; fi
   cache:
     paths: