fix: Use self.outPath to include submodules in Nix build

When Nix builds with .?submodules=1, using self.outPath ensures
that git submodules (like tree-sitter-nim) are included in the
source copied to the build sandbox.

Previously, src = ../../. only included files from the main repo,
causing 'tree-sitter-nim' dependency errors during cargo build.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: zah

nix/packages/default.nix

@@ -1,4 +1,4 @@
-{ inputs, ... }:
+{ inputs, self, ... }:
 {
   perSystem =
     {
@@ -9,7 +9,10 @@
     let
       inherit (pkgs) stdenv;
 
-      src = ../../.;
+      # Use self.outPath to get the flake source with submodules
+      # When building with .?submodules=1, Nix will fetch submodules
+      # and self.outPath will include them
+      src = self.outPath;
 
       # Import multiple Nim versions
       nimVersions = import ../nim-versions { inherit pkgs; };