Fix incorrect int to string conversion

This line breaks compilation under go 1.15

Author: jpcoenen

internals/cli/filemode/filemode.go

@@ -2,6 +2,7 @@
 package filemode
 
 import (
+	"fmt"
 	"os"
 	"strconv"
 
@@ -47,7 +48,7 @@ func (m *FileMode) Set(value string) error {
 
 // String implements the flag.Value interface.
 func (m FileMode) String() string {
-	return string(m)
+	return fmt.Sprint(m)
 }
 
 // FileMode returns the file mode as an os.FileMode.