cmd/go: fix list_constraints test on non-amd64 platforms

Test set GOOS to linux and darwin without setting GOARCH. darwin is
not a valid GOOS for all architectures we test.

Fixes #35976

Change-Id: I4da2ebcbf9ad52e07bcc1632b48fcfdbc49b1289
Reviewed-on: https://go-review.googlesource.com/c/go/+/209900
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/testdata/script/list_constraints.txt

@@ -3,6 +3,7 @@
 
 # Linux and cgo files should be included when building in that configuration.
 env GOOS=linux
+env GOARCH=amd64
 env CGO_ENABLED=1
 go list -f '{{range .GoFiles}}{{.}} {{end}}'
 stdout '^cgotag.go empty.go suffix_linux.go tag.go $'