iclouddrive: fix so created files are writable

At present any created file (eg through the touch command, copy, mount
etc) is read-only in iCloud.

This has been reported by users at
https://forum.rclone.org/t/icloud-and-file-editing-permissions/50659.

backend/iclouddrive/api/drive.go

@@ -631,7 +631,7 @@ func NewUpdateFileInfo() UpdateFileInfo {
 		FileFlags: FileFlags{
 			IsExecutable: true,
 			IsHidden:     false,
-			IsWritable:   false,
+			IsWritable:   true,
 		},
 	}
 }