fix #4521: api/tasks: replace upid as filename for task log downloads

fix #4521: api/tasks: replace upid as filename for task log downloads

previously the upid would just be used without a file extension when
downloading a task log. this lead to rather strange filenames that
appeared unfamiliar to users as the upid is not very prevalent in the
gui. set a proper file name based on the node name, worker type and a
time stamp instead. also add the ".log" file extension to indicate
that these files contain logs.

Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>