GitHub Action CI: Add Go source file format check (#6090)

https://github.com/XTLS/Xray-core/pull/6057#issuecomment-4364819830

And https://github.com/XTLS/Xray-core/pull/6149#issuecomment-4546876261

proxy/tun/tun_default.go

@@ -9,8 +9,7 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
 
-type DefaultTun struct {
-}
+type DefaultTun struct{}
 
 // DefaultTun implements Tun
 var _ Tun = (*DefaultTun)(nil)