Improve downloading artifacts for OSTree artifact cache

This fixes two issues: first, downloads were being put into the default
TMPDIR, which is an in-memory file system. When large artifacts are put
into /tmp such as gcc-devel, which is over 400MB, I found that my system
became unusable due to out of memory / swap death. Ideally we'd fix
Linux's memory management, but a more practical solution for now is to
download the files to the artifact cache directory. I've not noticed any
loss in speed from this change.

Second, temporary files should always cleaned up now, even when there
are errors during transfer or extraction.

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