ci: parallel build in manifest stage

.gitlab-ci.yml

@@ -189,7 +189,7 @@ target:x86-64:
 package:
   stage: package
   script:
-    - for branch in "stable" "beta" "nightly" "experimental"; do make -C gluon manifest GLUON_AUTOUPDATER_BRANCH="$branch" GLUON_AUTOUPDATER_ENABLED=1 GLUON_SITEDIR="$CI_PROJECT_DIR" FFS_BUILD_DATE=$(cat build.date) V=1; done
+    - for branch in "stable" "beta" "nightly" "experimental"; do make -C gluon manifest GLUON_AUTOUPDATER_BRANCH="$branch" GLUON_AUTOUPDATER_ENABLED=1 GLUON_SITEDIR="$CI_PROJECT_DIR" FFS_BUILD_DATE=$(cat build.date) V=1 -j`nproc`; 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: