chore: Use GitHub App authentication to generate "checkout tokens"

.github/workflows/build.yml

@@ -44,12 +44,18 @@ jobs:
         uses: actions/setup-python@v3
         with:
           python-version: "3.10"
+      - name: Generate Nillion Repo app token
+        id: generate-token
+        uses: actions/create-github-app-token@v1
+        with:
+          app-id: ${{ vars.NILLION_REPO_APP_ID }}
+          private-key: ${{ secrets.NILLION_REPO_PRIVATE_KEY }}
       - name: Checkout tools repo
         uses: actions/checkout@v4
         with:
           repository: NillionNetwork/nillion
           path: nillion
-          token: ${{ secrets.NILLION_TOKEN }}
+          token: ${{ steps.generate-token.outputs.token }}
       - name: Replace nillion's nada_dsl with this version
         uses: actions/checkout@v4
         with: