refactor: clarify GOOS detection (#39620)

Introduce `consts.IsWindows`, now it's clearer to see how Windows build works

modules/setting/setting.go

@@ -8,18 +8,16 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"runtime"
 	"strings"
 	"time"
 
+	"gitea.dev/modules/consts"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/optional"
 	"gitea.dev/modules/user"
 	"gitea.dev/modules/util"
 )
 
-const IsWindows = runtime.GOOS == "windows"
-
 var (
 	// AppVer is the version of the current build of Gitea. It is set in main.go from main.Version.
 	AppVer string
@@ -52,7 +50,7 @@ func init() {
 // This check is ignored under Windows since SSH remote login is not the main
 // method to login on Windows.
 func IsRunUserMatchCurrentUser(runUser string) (string, bool) {
-	if IsWindows || SSH.StartBuiltinServer {
+	if consts.IsWindows || SSH.StartBuiltinServer {
 		return "", true
 	}