Use a more actively-maintained nix image

On CI with the nixorg/nix:circleci image:

```
warning: unable to download 'https://hydra.iohk.io/nix-cache-info': SSL peer certificate or SSH remote key was not OK (60); retrying in 350 ms
```

hydra.iohk.io uses a Let's Encrypt certificate so this error is probably
caused by the ~3 year old image doesn't have "ISRG Root X1" in its trust roots
and the older CA certificate "DST Root CA X3" expired in October 2021.

Perhaps this more actively maintained nix image has had "ISRG Root X1" added
to it.