Add 'git checkout' to working state of repo

Author: kkawula

.github/workflows/checks.yml

@@ -197,6 +197,7 @@ jobs:
           options: --rm -v ${{ github.workspace }}:/apps
           run: |
             cd /apps/app-starknet
+            git checkout 9ff80d763235482860120a8350da8b032e3b6473
             cargo clean
             cargo ledger build nanox