Should we really open tempfile with truncate(false)?

[szokeasaurusrex]

We had to add .truncate(false) to our tempfile opening code in #1988 to fix a new Clippy lint. .truncate(false) is equivalent to the previous behavior, but it seems like it would make more sense to use truncate(true). However, when I tried truncate(true), some tests failed.

Perhaps, we should be using truncate(true), and we in fact have a bug here.

[szokeasaurusrex]

After investigating, I determined that truncate(false) provides the desired behavior, after all. I will prepare a PR to add comments to document why this is the case.