inko: unpin LLVM

Upstream is already using LLVM 18 and it builds fine with the default.

pkgs/by-name/in/inko/package.nix

···
   lib,
   rustPlatform,
   fetchFromGitHub,
-  llvm_17,
+  llvm,
   libffi,
   libz,
   libxml2,
···
   ];
 
   nativeBuildInputs = [
-    llvm_17
+    llvm
     makeWrapper
   ];