gitlab-ci: switch to multi-project pipeline for build stage

Instead of defining a custom job which mirrors the build result from the
manifest, use the GitLab multi-project feature to trigger a downstream
job in the manifest.

.gitlab-ci.yml

@@ -13,6 +13,9 @@ variables:
   CI_IMAGES: registry.gitlab.com/garz-fricke/yocto/infrastructure/ci-images
   # Include git submodules
   GIT_SUBMODULE_STRATEGY: recursive
+  # FIXME: due to a missing feature in GitLab we cannot use this variable in the build
+  # stage further down this file. If it ever changes, it has to be changed there too.
+  # (https://gitlab.com/gitlab-org/gitlab/-/issues/249583)
   MANIFEST_PROJECT: garz-fricke/yocto/manifest
   # The master branch is hardcoded here, because it cannot be determined automatically.
   # Has to be modified for new branches, e.g. new Yocto versions or fix releases.
@@ -84,21 +87,10 @@ build:
   stage: build
   rules:
     - if: $CI_MERGE_REQUEST_IID
-    - if: $CI_COMMIT_BRANCH == $MASTER_BRANCH
-  dependencies:
-    - integrate
-    - merge
-  tags:
-    - infrastructure
-  timeout: 60m
-  script:
-    - cd ${CI_PROJECT_DIR}
-    - echo "Manifest revision $(cat manifest_revision)"
-    - .gitlab-ci/check_pipeline_status.py
-        --gitlab-url=${CI_SERVER_URL}
-        --token=${GITBOT_TOKEN}
-        --project=${MANIFEST_PROJECT}
-        --commit=$(cat manifest_revision)
+  trigger:
+    project: garz-fricke/yocto/manifest
+    branch: "integrate/${CI_PROJECT_NAME}/${CI_COMMIT_REF_NAME}"
+    strategy: depend
 
 #---------------------------------------------------------------------------------------
 # Stage: check