Dunno if this is correct, there's other `hostPlatform.isDarwin`
conditionals which may be wrong.
Leaving the other conditionals to swift maintainers or people who are
interested in swift cross-compilation
```
$ nix repl --file . --system aarch64-darwin
nix-repl> pkgsCross.aarch64-multiplatform.swift
«derivation /nix/store/kqbaqwdfc9g0yvkq4ffrlqspranprlmy-swift-wrapper-aarch64-unknown-linux-gnu-5.8.drv»
nix-repl> pkgsCross.aarch64-multiplatform.buildPackages.swift
error:
error: expected a string but found null: null
at /home/artturin/nixgits/my-nixpkgs/.worktree/2/pkgs/development/compilers/swift/compiler/default.nix:586:11:
585| #WATCHOS_SIMULATOR
586| }.${targetPlatform.darwinPlatform}}
| ^
587|
```
276fe40ff3