Tails Installer logs error from subprocesses to
/tmp/liveusb-creator.log. That's a well-known name in a world-writable
location, so leaves room for some kinds of attacks e.g.
arbitrary file modification. The proposed branch fixes that.
Note that I did not build a Debian package, but tested it by patching
the code in a live Tails (and adding a line to creator.py so that it
logs even on success).
Assigning to anonym, since our usual other review'n'mergers are
apparently not available these days.
