fix(ci): add IMPORT_HOST_DIR stub to compose-idempotency env file

Docker Compose interpolates all variables in the full file even when
only a subset of services is requested. The backend service uses
IMPORT_HOST_DIR with :? (hard-required), causing the idempotency job
to abort before any container starts. A dummy path satisfies the parser;
the backend service is never started in this job so the path need not exist.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -269,6 +269,7 @@ jobs:
           MAIL_HOST=mailpit
           MAIL_PORT=1025
           APP_MAIL_FROM=noreply@local
+          IMPORT_HOST_DIR=/tmp/dummy-import
           EOF
 
       - name: Bring up minio