New comment by waynevanson on void-packages repository

https://github.com/void-linux/void-packages/pull/24723#issuecomment-691385975

Comment:
As documented in the [optional variables section of the manual](https://github.com/void-linux/void-packages/blob/master/Manual.md#optional-variables), host dependencies cannot be set to a version. [This comment on GitHub](https://github.com/purescript/spago/issues/686#issuecomment-691197535) indicates that we need GHC to be of version `8.4` in order for it to build `spago`.

So far my working solution is to download the GHC specified by spago. This runs using `stack build` during phase `do_build()`.
Should downloading GHC via stack happen at a different phase? If so, when?