build: don't conflate `-d:release` and "build job options"

There's currently tension in `config.nims` between what we want for a developer, and what we want for the build job. `-d:release` doesn't have to mean "what we use for releases that we publish".

Right now, `nimble build -d:release` won't work on Alpine Linux, because it doesn't have `musl-gcc`:

https://github.com/exercism/configlet/blob/a103ac4607d9eb6e4217cf9d324705fc43ad012c/config.nims#L10-L20

It's trivial to make it use `musl-gcc` only if it's available, but we don't want that to happen in the context of the build job.