fix: tvx: make extract-multiple support the FVM

[Stebalien]

Proposed Changes

1. Fix flushing to handle the case where the VM is an FVM.
2. Fix extract-multiple's code CID conversion.

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• CI is green

[arajasek]

Little bit of a "if you tell me it works, I ✔️" review, but it does seem reasonable.