rmt-cli mirror should use better temporary directory

[olafhering]

It seems that all files are downloaded to /tmp before they are moved to /usr/share/rmt/public/repo/.

Since the directory /usr/share/rmt/public/repo/ points to the actual storage (the RMT admin owns that symlink), it can easily happen that rmt-cli mirror will run into ENOSPC because /tmp may be too small.

I suggest to use /usr/share/rmt/public/repo/.tmp as temporary storage, unless TMPDIR is already set.