fix: ensure leann-core package is built on all platforms, not just Ubuntu

This fixes the issue where CI was installing leann-core from PyPI instead of
using locally built package with Python 3.9 compatibility fixes.

.github/workflows/build-reusable.yml

@@ -137,11 +137,9 @@ jobs:
       - name: Build packages
         run: |
           # Build core (platform independent)
-          if [[ "${{ matrix.os }}" == ubuntu-* ]]; then
-            cd packages/leann-core
-            uv build
-            cd ../..
-          fi
+          cd packages/leann-core
+          uv build
+          cd ../..
 
           # Build HNSW backend
           cd packages/leann-backend-hnsw