cmd/go/internal/clean: add logging to help debug openbsd flakes
This change adds extra logging in the case where there's an error
removing all the files in the gomodcache using modfetch.RemoveAll.
It logs the names of the files found in GOMODCACHE as well as their
modes. The modes are included because they should all be writable by the
time we call robustio.RemoveAll.
For #68087
Change-Id: Id9ae68bf6a3392baf88ec002d08fed1faf525927
Reviewed-on: https://go-review.googlesource.com/c/go/+/658816 Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Matloob <matloob@golang.org>