adjust whitespace

Summary: Oof, apparently we're still not quite done bikeshedding how this appears in the terminal...
I didn't realize someone changed these "still going" messages back into hardcoded \r's.
If we still want the spinner to appear normal when piped through arc build (and
the reason we don't just exec from arc build is so that we can still do timeouts
from arc and --rage if the server is unresponsive) we can't use Tty.clear_line,
so just print extra space after the spinner to prevent ghost spinners when the line
length changes.

these false positives are really persistent...

Reviewed By: @aryx

Differential Revision: D1452303