fix(ci): bump libdatadog to include spawn_worker review fixes

Update libdatadog submodule from 249be4639 to 53735b1e7, which adds:
- stack-allocated path (no heap post-fork)
- sendfile return value check
- fork-safe atexit handler (recomputes path from getpid())
- atexit registered in both O_EXCL success and orphan-reuse branches
- presence-only env var check (!val.is_null())

Author: Leiyks