Run:
ninja -C out/cros_official libassistant.so -j500 -l10 -k0 # 1st time
ninja -C out/cros_official libassistant.so -j500 -l10 -k0 # 2nd time

Expected:

ninja: no work to do.

Actual:

ninja: Entering directory `out/crosofficial'
[1/1] ACTION //chromeos/assistant/internal:build_libassistant(//build/toolchain/linux:clang_x64)
________ running '/usr/bin/python third_party/depot_tools/download_from_google_storage.py ...
________ running '/usr/bin/python third_party/depot_tools/download_from_google_storage.py ...
________ running 'build/libassistant/install-sysroot.py --running-as-hook' in ...
Done. Made 468 targets from 201 files in 235ms
ninja: Entering directory `obj/chromeos/assistant/internal'
ninja: no work to do.

^ This took 6 seconds.