swift: Fix eval failure error: expected a string but found null: null

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|
```

pkgs/development/compilers/swift/compiler/default.nix

@@ -541,7 +541,7 @@ in stdenv.mkDerivation {
     ";
     buildProject lldb llvm-project/lldb
 
-    ${lib.optionalString stdenv.hostPlatform.isDarwin ''
+    ${lib.optionalString stdenv.targetPlatform.isDarwin ''
     # Need to do a standalone build of concurrency for Darwin back deployment.
     # Based on: utils/swift_build_support/swift_build_support/products/backdeployconcurrency.py
     cmakeFlags="