Allow specifying custom tempdir for files.download

[scy]

This introduces a new argument to the files.download operation: temp_dir, which allows the user to specify a custom temporary directory for downloading into.

I know that TEMP_DIR exists as an (undocumented) config option, but since Host._get_temp_directory is memoized, it's not feasible to set this option only for certain operations.

My use case is downloading very large files (> 100 GB) into specific volumes on the server, which are backed by different filesystems and have some constraints regarding free space. The default of downloading everything into /tmp (or whatever) doesn't work because it would fill up the root partition before the download even finishes, but setting TEMP_DIR to different directories throughout a single deploy is also not really possible without resorting to hacks, afaics.

To allow customizing the temporary directory, Host.get_temp_filename also gained a (keyword-only, optional) new argument temp_directory.

[scy]

The failing tests are unrelated to my changes.