cmd/go: log failures to remove workdir

Failures here don't otherwise affect the build, but they do cause a
slow file leak in the user's temp directory. The user deserves at
least a cursory warning that something may be amiss.

Updates #30789

Change-Id: Id0e72b1967e7f7c88cdc94d532554496653f264b
Reviewed-on: https://go-review.googlesource.com/c/go/+/171764
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/work/action.go

@@ -242,7 +242,11 @@ func (b *Builder) Init() {
 		}
 		if !cfg.BuildWork {
 			workdir := b.WorkDir
-			base.AtExit(func() { os.RemoveAll(workdir) })
+			base.AtExit(func() {
+				if err := os.RemoveAll(workdir); err != nil {
+					fmt.Fprintf(os.Stderr, "go: failed to remove work dir: %s\n", err)
+				}
+			})
 		}
 	}