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

[ee7]

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