crosdl.py: fix file name encoding bug

Simple change to make crosdl.py convert the file_name variable to utf-8
before trying to use it to make a path.

BUG=None
TEST=crosdl.py -p <Board> -c dev -t --to_ip <IP> -b <Build>

Change-Id: If8da19482a6a8e1a4442faffca4a01b78006fe84
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/2001125
Tested-by: Brendan Hansknecht <bhansknecht@chromium.org>
Reviewed-by: Katherine Threlkeld <kathrelkeld@chromium.org>
Commit-Queue: Brendan Hansknecht <bhansknecht@chromium.org>
1 file changed