cmd/go: fix nocgo builder

Change-Id: I0f167d9769d9b6b9888c13fcb48e538fc87aa1b7
Reviewed-on: https://go-review.googlesource.com/12240
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/go_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"flag"
 	"fmt"
+	"go/build"
 	"go/format"
 	"internal/testenv"
 	"io/ioutil"
@@ -2106,6 +2107,10 @@ func TestGoTestRaceInstallCgo(t *testing.T) {
 		t.Skip("no race detector on %s", sys)
 	}
 
+	if !build.Default.CgoEnabled {
+		t.Skip("no race detector without cgo")
+	}
+
 	// golang.org/issue/10500.
 	// This used to install a race-enabled cgo.
 	tg := testgo(t)