Resolution order (highest precedence first):
1. exported MIGRATE_PIPELINE_ROOT (e.g. from a Jenkinsfile)
2. MIGRATE_PIPELINE_ROOT set by .envrc in this directory
3. derived from the script location (three levels up)
The script always lives at <root>/qt-migration-legatus/qt6-after/
qt6-migrated-legatus, so the pipeline root is three levels up. This
makes the script work in a fresh Jenkins checkout with no environment
setup at all, and .envrc is no longer required.