Use LLVM 17 by default in flake (#4370)

@ehildenb bumped into llvm/llvm-project#60972
again when [updating KEVM
tests](https://github.com/runtimeverification/evm-semantics/pull/); at
the time I fixed the bug originally it wasn't yet in a released version
of LLVM we could use, and so we had to just ignore the issue.

Now that LLVM 17 is out, we should update K to use it by default where
possible; this PR does so for the Nix flake build of K.

flake.nix

@@ -16,7 +16,7 @@
     let
       allOverlays = [
         (_: _: {
-          llvm-version = 15;
+          llvm-version = 17;
           llvm-backend-build-type = "Release";
         })
         llvm-backend.overlays.default