cmd/bloom: actually, always use the same temp filename.

cmd/bloom: actually, always use the same temp filename.

There's no reason to use a different temp filename every time, since we're
going to just be overwriting the same output file anyhow.  And if we got
interrupted, we left the temp file lying around.  Let's just always use the
same temp filename, which means if we get interrupted, we'll clean it up
next time.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>