Support for cross-compiling GHC has been merged into Nixpkgs, so it is now possible to boot RISC-V GHC by cross-compiling from a supported platform.
fgaz this might have broken your ShellCheck fix, because GHC now claims that it supports all platforms even though it cannot be evaluated on exotic platforms unless a boot compiler is provided.

Next step: adding boot GHC binaries for RISC-V (and maybe some other platforms) to remove the need to cross-compile; these binaries can now be built entirely from Nixpkgs and upstream binaries.