perf: use self-hosted runner for matrix-setup job

- Reduces matrix generation time from ~30s to ~3s
- No VM provisioning overhead
- Perfect for tiny, safe, read-only jobs like JSON generation

.github/workflows/xahau-ga-nix.yml

@@ -12,7 +12,7 @@ concurrency:
 
 jobs:
   matrix-setup:
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, vanity]
     outputs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
     steps: