It's tiny, but we dont' really need it so we might as well trim our
dependencies down a bit.

I'm doing this in CI instead of in the Cargo.toml file because I don't
want to force users of newer rustc versions to use old versions of
`once_cell`. Ironically, I only depend on `once_cell` to support ancient
rust versions in the first place.

This adds a feature making it possible to disable the immediate deletion
of unnamed temporary files on windows (introduced in 461369f).
Hopefully, this will fix #339. If so, I'll revert 461369f entirely.