cmd/go: clear cmd cache to avoid duplicate loads errors

go get -u all command updates all packages including standard
commands. We need to get commands evicted from their cache to
avoid loading old versions of the packages evicted from the
packages cache.

Fixes #14444

Change-Id: Icd581a26e1db34ca634aba595fed62b097094c2f
Reviewed-on: https://go-review.googlesource.com/19899


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/22040


Reviewed-by: Andrew Gerrand <adg@golang.org>