cmd/go: clean up "cover" debug output
Currently, the two places that run the cover tool manually construct a
terse command description. However, this actually prevents Builder.run
from constructing a more detailed description from the actual command
being run. Fix this by passing "" as the description to get the
default behavior.
Change-Id: I27d42cb1fda9bba70c631dc43417a03b8bddec92
Reviewed-on: https://go-review.googlesource.com/c/go/+/534157
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>