feat: worker: Ensure tempdir exists on startup

[magik6k]

Related Issues

This is something I occasionally run into in my setup when starting workers with custom TMPDIR, which on some of my systems disappears after reboot.

If not caught this would cause tasks like AddPiece to fail with a somewhat crypic error like

Caused by:
    0: failed to create data store
    1: No such file or directory (os error 2)

Proposed Changes

Make UX slightly better when the tempdir doesn't exist.

Additional Info

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
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[shrenujbansal]

Where do we have cleanup for this temp dir?

[fridrik01]

Where do we have cleanup for this temp dir?

I don't think we need to as this is simply making sure that future calls to os.TempDir() will not fail when overriding $TMPDIR.

[fridrik01]

LGTM