cmd/go: ignore /tmp/go.mod

Two different people have created /tmp/go.mod for experimentation
and then had other tests that create fresh work directories
below /tmp fail unexpectedly because the go command finds
/tmp/go.mod. Refuse to use /tmp/go.mod. /tmp/anything/go.mod is fine.

Fixes #26708.

Change-Id: I2a4f61ea63099cff59fbf9e8798e5dcefefd5557
Reviewed-on: https://go-review.googlesource.com/129063


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>