chore: add S2_API_KEY to dockerized workflow

[robe-ai2]

In order to run:

uv run astabench eval --solver generate --model openai/gpt-4o --limit 3 --split validation --ignore-git --retry-attempts 1 --sandbox=local

via make shell, I needed to pass through S2_API_KEY

[regan-huff]

I have been running this by defining S2_API_KEY in an .env file, which I believe gets added here in the Makefile:

ifneq ("$(wildcard .env)","")
  ENV_ARGS += --env-file .env
endif

Do you think it is important to call this out more explicitly in the Makefile?

[robe-ai2]

@regan-huff I think in docker-land it's idiomatic/known enough that -e envvars take precedence over .env that the comments in the makefile are sufficient? previously in #13 i was trying to support both options in both local & ci envs ... i think that S2_API_KEY is a core-enough dependency that it should be lifted up explicitly to the top of the Makefile as an option so that people who want to use non-.env variable injection can use it too