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

main/confloader/confloader.go

@@ -12,9 +12,7 @@ type (
 	configFileLoader func(string) (io.Reader, error)
 )
 
-var (
-	EffectiveConfigFileLoader configFileLoader
-)
+var EffectiveConfigFileLoader configFileLoader
 
 // LoadConfig reads from a path/url/stdin
 // actual work is in external module