cmd/cue/cmd: always print newline at end of eval
format.Node doesn't print a newline after
a single expression.
Change-Id: I87220e0c2a290ffd7618c4633329d3ef614df834
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3521
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>