Variables in trigger job includes are not expanded

[Frederik Schubert]

Minimal .gitlab-ci.yml illustrating the issue

---
.common_trigger:
  stage: triggers
  trigger:
    include:
      - local: $PROJECT_DIR/.gitlab-ci.yml

trigger_project:
  variables:
    PROJECT_DIR: path/to/project
  extends: .common_trigger

Expected behavior
The variables at this line should be expanded using the set of global and job variables.

Host information
Ubuntu
gitlab-ci-local 4.57.0

Containerd binary
I am using docker.

Additional context
Add any other context about the problem here.