cody-bench: exit the process after finishing

[olafurpg]

When running the tool locally, I noticed that it was hanging after completing the eval run. Probably, what's happening is that there's some background promise that hasn't completed running. I didn't experience this when I originally created the tool so it looks like a regression. This PR "fixes" the problem by calling process.exit(0), which is good enough to unblock usage of the tool

Test plan

Clone sourcegraph/cody-leaderboard next to the cody repo. Run this from the Cody repo

pnpm install
pnpm build
cd agent
pnpm agent:skip-root-build cody-bench --evaluation-config ../../cody-leaderboard/fix-bench.json