refactor: clarify GOOS detection (#39620)

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

modules/user/user.go

@@ -6,19 +6,20 @@ package user
 import (
 	"os"
 	"os/user"
-	"runtime"
 	"strings"
+
+	"gitea.dev/modules/consts"
 )
 
-// CurrentUsername return current login OS user name
+// CurrentUsername return current login OS username
 func CurrentUsername() string {
 	userinfo, err := user.Current()
 	if err != nil {
 		return fallbackCurrentUsername()
 	}
 	username := userinfo.Username
-	if runtime.GOOS == "windows" {
-		parts := strings.Split(username, "\\")
+	if consts.IsWindows {
+		parts := strings.Split(username, "\\") // remove domain if present
 		username = parts[len(parts)-1]
 	}
 	return username

modules/user/user_test.go

@@ -5,10 +5,11 @@ package user
 
 import (
 	"os/exec"
-	"runtime"
 	"strings"
 	"testing"
 
+	"gitea.dev/modules/consts"
+
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -26,7 +27,7 @@ func TestCurrentUsername(t *testing.T) {
 	require.NotEmpty(t, user)
 
 	// Windows whoami is weird, so just skip remaining tests
-	if runtime.GOOS == "windows" {
+	if consts.IsWindows {
 		t.Skip("skipped test because of weird whoami on Windows")
 	}
 	whoami, err := getWhoamiOutput()