cmd/go/internal/modfetch: always allow Stat for the current toolchain to succeed

cmd/go/internal/modfetch: always allow Stat for the current toolchain to succeed

This fixes a failure mode observed in TestScript/gotoolchain_version
when building go1.21rc3.

Updates #61259.

Change-Id: Ifb14a5b94b687adea7a26c1155591e3ae75b7a62
Reviewed-on: https://go-review.googlesource.com/c/go/+/509217
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>