ci: fix /homeless-shelter error in container builds

- Set HOME=/root explicitly to prevent Nix installer from creating /homeless-shelter
- Add Prepare environment step to clean up before Nix install
- Disable sandbox explicitly (not available in Docker containers)

.forgejo/workflows/ci.yml

@@ -7,8 +7,15 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    env:
+      HOME: /root
 
     steps:
+      - name: Prepare environment
+        run: |
+          mkdir -p "$HOME"
+          rm -rf /homeless-shelter
+
       - name: Checkout
         uses: actions/checkout@v6
 
@@ -16,6 +23,8 @@ jobs:
         uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable
+          extra_nix_config: |
+            sandbox = false
 
       - name: Enable flakes
         run: |