[Release] Make download_rc_binaries.py less frustrating

### Describe the enhancement requested

- Use `--retry-all-errors` in cURL, or do the retry in Python instead, to avoid 'connection reset by peer'
- Skip files already downloaded so that we don't have a [painter's algorithm](https://en.wikichip.org/wiki/schlemiel_the_painter%27s_algorithm) when we retry validation

### Component(s)

Release