build(nix): use stdenv.hostPlatform.isDarwin in atuin.nix (atuinsh/atuin#4391)

nixpkgs now warns on `stdenv.isDarwin`, so anyone consuming the flake sees this on every evaluation:

    evaluation warning: stdenv.isDarwin is deprecated, use stdenv.hostPlatform.isDarwin instead

This switches the libiconv guard to the `hostPlatform` spelling, which the `postFixup` guard a few lines down already uses. Same idea as atuinsh/atuin#3006.

The resulting derivations are identical apart from the source hash: with store hashes stripped, the env matches before and after on aarch64-darwin and x86_64-linux, and Darwin still gets libiconv. `nix fmt -- --ci` and `nix flake check --no-build` pass, and I built it and ran `atuin --version` on aarch64-darwin. I haven't built it on Linux.

## Checks
- [x] I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
- [x] I have checked that there are no existing pull requests for the same thing

atuin.nix

@@ -28,7 +28,7 @@ let
       pkg-config
     ];
 
-    buildInputs = [ openssl ] ++ lib.optionals stdenv.isDarwin [ libiconv ];
+    buildInputs = [ openssl ] ++ lib.optionals stdenv.hostPlatform.isDarwin [ libiconv ];
 
     OPENSSL_NO_VENDOR = 1;