cmd/go: log more information about leaked temp files

cmd/go: log more information about leaked temp files

Previously, we were only logging the top-level names of leaked
directories, which doesn't provide much information for debugging.

For #55260.

Change-Id: I845d158135d67b5d7fdeb16ab7031a061535e643
Reviewed-on: https://go-review.googlesource.com/c/go/+/479055
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>