internal/flags: fix "flag redefined" bug for alias on custom flags (#30796)

This change fixes a bug on the `DirectoryFlag` and the `BigFlag`, which would trigger a `panic` with the message "flag redefined" in case an alias was added to such a flag.

internal/flags/flags.go

@@ -90,7 +90,7 @@ func (f *DirectoryFlag) Apply(set *flag.FlagSet) error {
 		}
 	}
 	eachName(f, func(name string) {
-		set.Var(&f.Value, f.Name, f.Usage)
+		set.Var(&f.Value, name, f.Usage)
 	})
 	return nil
 }
@@ -172,7 +172,7 @@ func (f *BigFlag) Apply(set *flag.FlagSet) error {
 	}
 	eachName(f, func(name string) {
 		f.Value = new(big.Int)
-		set.Var((*bigValue)(f.Value), f.Name, f.Usage)
+		set.Var((*bigValue)(f.Value), name, f.Usage)
 	})
 	return nil
 }