noxfile.py ignores posargs

[Mahhheshh]

CONTRIBUTING.md mentions a way to run a single test subset; but in the noxfile.py, the posargs are set to empty. This causes any arguments passed after -- to be discarded.
reference;

braintrust-sdk-python/py/noxfile.py

Line 389 in 9548ae3

common_args = ["--disable-vcr"] if "--disable-vcr" in session.posargs else []

braintrust-sdk-python/py/noxfile.py

Line 411 in 9548ae3

session.run(*test_args, *common_args, env=env)