ci: remove /homeless-shelter right before build

- Prep step runs after install-nix Pre step (actions order), so useless there
- Move cleanup to a dedicated step directly before nix build

.forgejo/workflows/ci.yml

@@ -11,11 +11,6 @@ jobs:
       HOME: /root
 
     steps:
-      - name: Prepare environment
-        run: |
-          mkdir -p "$HOME"
-          rm -rf /homeless-shelter
-
       - name: Checkout
         uses: actions/checkout@v6
 
@@ -42,6 +37,9 @@ jobs:
       - name: Flake check
         run: nix flake check
 
+      - name: Remove homeless-shelter
+        run: rm -rf /homeless-shelter
+
       - name: Build all hosts
         run: |
           nix flake show --json | jq -r '