cmd/go: support 'go run cmd@version'

'go run' can now build a command at a specific version in module-aware
mode, ignoring the go.mod file in the current directory if there is one.

For #42088

Change-Id: I0bd9bcbe40c0442a268cd1cc315a8a2cbb5adeee
Reviewed-on: https://go-review.googlesource.com/c/go/+/310074


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>