ci: could use `iffy/install-nim` for every Nim version

Historically, the track has used `alaviss/setup-nim` to run tests with nightly Nim releases, and `iffy/install-nim` for stable Nim releases:

https://github.com/exercism/nim/blob/8fa077f680bc078ee29062dd5ea3d9422c647d5e/.github/workflows/exercises.yml#L32-L45

But I believe that `iffy/install-nim` has the ability to use the Nim nightlies too, with something like:

```
nightly:https://github.com/nim-lang/nightlies/releases/tag/latest-version-2-0
```