runtime: use CleanCmdEnv in TestRuntimePanic

This makes TestRuntimePanic keep most of the existing environment,
just as the other runtime tests do.

Change-Id: I7944abfeee292d41716dca14483134a50d75f081
Reviewed-on: https://go-review.googlesource.com/135376
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/crash_test.go

@@ -686,7 +686,7 @@ func init() {
 
 func TestRuntimePanic(t *testing.T) {
 	testenv.MustHaveExec(t)
-	cmd := exec.Command(os.Args[0], "-test.run=TestRuntimePanic")
+	cmd := testenv.CleanCmdEnv(exec.Command(os.Args[0], "-test.run=TestRuntimePanic"))
 	cmd.Env = append(cmd.Env, "GO_TEST_RUNTIME_PANIC=1")
 	out, err := cmd.CombinedOutput()
 	t.Logf("%s", out)