feat: add build step for contracts in Slither CI workflow

Author: Hakob23

.github/workflows/slither.yml

@@ -26,6 +26,9 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      - name: Build contracts
+        run: forge build
+
       - name: Run Slither
         uses: crytic/slither-action@f197989dea5b53e986d0f88c60a034ddd77ec9a8 # v0.4.0
         id: slither