Sandboxed dirs seem to interfere with download resumption

Now that pacman downloads partial files into download-*/ subdirectories, due to the subdirectory name being randomized it is no longer able to resume an interrupted download. For example, Ctrl-C'ing at the download stage in the middle of a ~100 MB package file will leave a 50 MB .part in download-aaaaa/ but the next invocation will start a fresh download into download-bbbbb/.

(The lack of resumption means that an existing .part file in those "old" subdirectories won't be cleaned up, either, leaving around junk that even pacman -Scc won't delete.)