Github API rate limited exceeded
I use choosenim in several Github Actions and am consistently exceeding the Github API Rate limits.  I'll admit that this seems to me like a Github problem, not a choosenim problem, but I don't know how to convince Github to fix it.

It manifests as this error:

```
Error: unhandled exception: download.nim(342, 14) `responseCode == 200` Expected HTTP code 200 got 403 for https://api.github.com/repos/nim-lang/nightlies/releases [AssertionError]
```

I've verified that the rate limits are the problem with:

```
curl https://api.github.com/rate_limit
  shell: /bin/bash -e {0}
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
{
  "resources": {
    "core": {
      "limit": 60,
      "remaining": 0,
      "reset": 1595258445
    },
    "graphql": {
      "limit": 0,
      "remaining": 0,
      "reset": 1595258786
    },
    "integration_manifest": {
      "limit": 5000,
      "remaining": 5000,
      "reset": 1595258786
    },
    "search": {
      "limit": 10,
      "remaining": 10,
      "reset": 1595255246
    }
  },
  "rate": {
    "limit": 60,
    "remaining": 0,
    "reset": 1595258445
  }
}
```

If choosenim allowed me to provide Github API credentials to be used in Github API requests, I could make those requests under my own account (rather than the anonymous rate).  I'll see if I can make a PR, but maybe someone else could make it much quicker.