ci: free disk space before Nix build to prevent ENOSPC on NVIDIA/CUDA packages

GitHub Actions free runners have ~14 GB total; removing unused toolchains
(dotnet, android, CodeQL, boost) reclaims ~8-10 GB needed for the full
NixOS system build.

https://claude.ai/code/session_01JwjdjwvgMGW4TcpYi4MZ6n

.github/workflows/check.yml

@@ -14,6 +14,14 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Free disk space
+        run: |
+          sudo rm -rf /usr/share/dotnet /usr/share/swift /usr/local/lib/android \
+            /opt/ghc /opt/hostedtoolcache/CodeQL /usr/local/share/boost \
+            "$AGENT_TOOLSDIRECTORY"
+          sudo docker image prune --all --force || true
+          df -h
+
       - name: Install Nix
         uses: cachix/install-nix-action@v27
         with: