Hardcode version in flake.nix

The git fetcher doesn't provide gitDescribe, so the Nix build was
falling through to shortRev and producing commit-hash versions instead
of tag-based ones.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

flake.nix

@@ -10,7 +10,7 @@
     let
       system = "x86_64-linux";
       pkgs = nixpkgs.legacyPackages.${system};
-      version = pkgs.lib.removePrefix "v" (self.gitDescribe or self.shortRev or self.dirtyShortRev or "unknown");
+      version = "0.8.3";
     in
     {
       packages.${system} = {