net/tlsdial,tstest,version: use go command from $PATH

Go now includes the GOROOT bin directory in $PATH while running tests
and generate, so it is no longer necessary to construct a path using
runtime.GOROOT().

Fixes #6689

Signed-off-by: James Tucker <james@tailscale.com>

tstest/archtest/qemu_test.go

@@ -11,8 +11,6 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
-	"path/filepath"
-	"runtime"
 	"strings"
 	"testing"
 
@@ -53,7 +51,7 @@ func TestInQemu(t *testing.T) {
 				}
 				t.Logf("using %v", look)
 			}
-			cmd := exec.Command(filepath.Join(runtime.GOROOT(), "bin", "go"),
+			cmd := exec.Command("go",
 				"test",
 				"--exec="+execVia,
 				"-v",