Pin Slither to 0.9.1 (#3949)

.github/workflows/checks.yml

@@ -78,6 +78,8 @@ jobs:
         uses: ./.github/actions/setup
       - run: rm foundry.toml
       - uses: crytic/slither-action@v0.2.0
+        with:
+          slither-version: 0.9.1
 
   codespell:
     if: github.repository != 'OpenZeppelin/openzeppelin-contracts-upgradeable'