I think you need clarify first what you want to do. It looks to me that you are trying to use two things that don’t really fit well together, so what is your ultimate goal there?
- haskell-flake is an abstraction layer built on top of nixpkgs (?) which will enable you to benefit from Nixpkgs’ prebuilt GHCs and Hackage packages for building your packages as well as development environments.
- ghc-wasm-meta provides a flake that allows you to use prebuilt binaries from GHC HQ’s CI of a GHC with the wasm backend. This works largely independently of the Nixpkgs Haskell infrastructure. Nixpkgs itself doesn’t provide a GHC with a wasm backend, since it is a nightmare to package.