Avoid deleting existing artifacts when ignoring hashes.

[nhz2]

Followup to #3764
Fixes bug reported in #3643 (comment)

This PR modifies download_artifact to avoid calling mv(src, dst; force=true) where src and dst are both possible valid artifacts.

The main steps of download_artifact are

1. Return true if the artifact already exists.
2. Determine the expected artifact path and parent directory.
3. Make a temporary directory.
4. download_verify_unpack into the temporary directory.
5. Calculate the tree hash of the temporary directory.
6. If the tree hash doesn't match, either throw an error or log an error, depending on the enviroment.
7. Atomically move the temporary directory to the expected path.
8. Try to clean up the temporary directory.

If steps 4 to 7 throw an error, that error gets returned, otherwise true is returned.

[IanButterworth]

@staticfloat does this look ok to you?

[IanButterworth]

Given you have a reproducer in #3643 (comment) could that be added as a test?

[nhz2]

Yes, I'll try and add a test. I should be able to reproduce the issue on Linux using the right environment variables and artifacts.