cmd/go: overwrite program name with full path

If the program path is resolved, replace the first argument of the
exec.Cmd, which is the bare program name with the resolved path.

Change-Id: I92cf5e6f4bb7c8fef9b59f5eab963f4e75b90d07
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/957908


Reviewed-by: Katie Hockman <katiehockman@google.com>
Reviewed-by: Russ Cox <rsc@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/284784


Trust: Roland Shoemaker <roland@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>