../test: set GOPATH in nosplit.go

This test invokes 'go build', so in module mode it needs a module
cache to guard edits to go.mod.

Fixes #30776

Change-Id: I89ebef1fad718247e7f972cd830e31d6f4a83e4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/167085
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/nosplit.go

@@ -217,6 +217,7 @@ func main() {
 		return
 	}
 	defer os.RemoveAll(dir)
+	os.Setenv("GOPATH", filepath.Join(dir, "_gopath"))
 
 	if err := ioutil.WriteFile(filepath.Join(dir, "go.mod"), []byte("module go-test-nosplit\n"), 0666); err != nil {
 		log.Panic(err)