cmd/go: retry RemoveAll(workdir) for up to 500ms

On some configurations of Windows, directories containing executable
files may be locked for a while after the executable exits (perhaps
due to antivirus scans?). It's probably worth a little extra latency
on exit to avoid filling up the user's temporary directory with leaked
files.

Updates #30789

Change-Id: Iae7fcdd07fb9ecfb05967cfe0c8833db646d2f85
Reviewed-on: https://go-review.googlesource.com/c/go/+/172337


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>