Fix: create compile nodes without checking installed_libs at DAG time
On a fresh cache, build layers don't exist yet when the compile DAG
is constructed, so scan_libs finds nothing. Moved the installed_libs
check to execution time (in compile_package) where the build layer
is guaranteed to exist.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>