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/iosdeps/iosdeps_test.go

@@ -13,13 +13,11 @@ import (
 	"encoding/json"
 	"os"
 	"os/exec"
-	"path/filepath"
-	"runtime"
 	"testing"
 )
 
 func TestDeps(t *testing.T) {
-	cmd := exec.Command(filepath.Join(runtime.GOROOT(), "bin", "go"), "list", "-json", ".")
+	cmd := exec.Command("go", "list", "-json", ".")
 	cmd.Env = append(os.Environ(), "GOOS=ios", "GOARCH=arm64")
 	out, err := cmd.Output()
 	if err != nil {