mistral-rs: make free again, refactor (#398403)

pkgs/by-name/mi/mistral-rs/package.nix

···
 }:
 
 let
+  inherit (stdenv) hostPlatform;
+
   accelIsValid = builtins.elem acceleration [
     null
     false
···
   metalSupport =
     assert accelIsValid;
     (acceleration == "metal")
-    || (stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isAarch64 && (acceleration == null));
+    || (hostPlatform.isDarwin && hostPlatform.isAarch64 && (acceleration == null));
 
 in
 rustPlatform.buildRustPackage (finalAttrs: {
···
   buildFeatures =
     lib.optionals cudaSupport [ "cuda" ]
     ++ lib.optionals mklSupport [ "mkl" ]
-    ++ lib.optionals (stdenv.hostPlatform.isDarwin && metalSupport) [ "metal" ];
+    ++ lib.optionals (hostPlatform.isDarwin && metalSupport) [ "metal" ];
 
   env =
     {
···
       CUDA_TOOLKIT_ROOT_DIR = lib.getDev cudaPackages.cuda_cudart;
     });
 
-  appendRunpaths = [
+  appendRunpaths = lib.optionals cudaSupport [
     (lib.makeLibraryPath [
       cudaPackages.libcublas
       cudaPackages.libcurand
···
   # swagger-ui will once more be copied in the target directory during the check phase
   # Not deleting the existing unpacked archive leads to a `PermissionDenied` error
   preCheck = ''
-    rm -rf target/${stdenv.hostPlatform.config}/release/build/
+    rm -rf target/${hostPlatform.config}/release/build/
   '';
 
   # Prevent checkFeatures from inheriting buildFeatures because
···
     tests = {
       version = testers.testVersion { package = mistral-rs; };
 
-      withMkl = lib.optionalAttrs (stdenv.hostPlatform == "x86_64-linux") (
+      withMkl = lib.optionalAttrs (hostPlatform.isLinux && hostPlatform.isx86_64) (
         mistral-rs.override { acceleration = "mkl"; }
       );
-      withCuda = lib.optionalAttrs stdenv.hostPlatform.isLinux (
-        mistral-rs.override { acceleration = "cuda"; }
-      );
-      withMetal = lib.optionalAttrs (stdenv.hostPlatform == "aarch64-darwin") (
+      withCuda = lib.optionalAttrs hostPlatform.isLinux (mistral-rs.override { acceleration = "cuda"; });
+      withMetal = lib.optionalAttrs (hostPlatform.isDarwin && hostPlatform.isAarch64) (
         mistral-rs.override { acceleration = "metal"; }
       );
     };