Use more explicit octal representation for clarity

My editor's autoformat started doing this:

```
-       f, err := fs.OpenFile(inputJSONPath, os.O_CREATE|os.O_WRONLY|os.O_EXCL, 0644)
+       f, err := fs.OpenFile(inputJSONPath, os.O_CREATE|os.O_WRONLY|os.O_EXCL, 0o644)
```

The reasoning is guess is that 0oNNN is more explicit and less likely to be misinterpreted. Let's change them all consistently in one neat PR.