ci-package-builder: Fix concurrent tag-release jobs

When landing two release MRs on the same project they may end up running their tag-release jobs at the same time. This can lead to conflicts:

• job 1: pulls the pristine-lfs-source branch
• job 2: pulls the pristine-lfs-source branch
• job 1: commit new sources to the pristine-lfs-source branch
• job 2: commit new sources to the pristine-lfs-source branch
• job 1: pushes new HEAD for pristine-lfs-source
• job 2: fails to pushes new HEAD for pristine-lfs-source since job 1 already updated it

See https://gitlab.apertis.org/pkg/base-files/-/jobs/579753 as an example of failed job.

ci-package-builder.yml

@@ -130,6 +130,8 @@ tag-release:
     GIT_STRATEGY: clone
     RELEASE: 1
     PRISTINE_SOURCE: pristine-lfs-source
+  # avoid conflicts when concurrently trying to commit stuff to the shared $PRISTINE_SOURCE branch
+  resource_group: $CI_PROJECT_ID
   stage: release
   tags:
     - lightweight