cmd/go: always include action mode in trace name

cmd/go: always include action mode in trace name

For actions with no package, the title "Executing action" is extremely
vague. Add the action mode so that there is some differentiation.

Change-Id: If6dcf81c7cd1f19a9532e56dd9f88abd1182ea97
Reviewed-on: https://go-review.googlesource.com/c/go/+/567936
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>