cmd/go/testdata: don't set GOPATH=NUL in test

cmd/go/testdata: don't set GOPATH=NUL in test

An upcoming change to the filepath package to make IsAbs("NUL")==false
on Windows will cause this test to fail, since it sets GOPATH=NUL and
GOPATH must be an absolute path.

Set GOPATH to the name of a text file instead. (The intent is that GOPATH
be set to a path that is not writable.)

For #56217.

Change-Id: I18e645fe11547d02d1a2e0e580085e6348c4009a
Reviewed-on: https://go-review.googlesource.com/c/go/+/448896
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>