packaging: Unbork win shells with unavailable dependencies

Makes the cross-x86_64-w64-mingw32 devshell slightly less
broken. It still needs a bit of massaging to function, but
that's much less cumbersome now that the generic machinery
with genericClosure that evaluates drvPath doesn't barf on
unavailable packages.

src/kaitai-struct-checks/package.nix

@@ -65,6 +65,6 @@ mkMesonDerivation (finalAttrs: {
   '';
 
   meta = {
-    platforms = lib.platforms.all;
+    platforms = lib.platforms.unix;
   };
 })

src/perl/package.nix

@@ -74,5 +74,9 @@ perl.pkgs.toPerlModule (
     ];
 
     strictDeps = false;
+
+    meta = {
+      platforms = lib.platforms.unix;
+    };
   })
 )