chore: Refer to renamed secrets in build.yml (#56)

This commit changes the reference to the app ID and private key values
used to generate app tokens used to close the nillion repo. This change
reflects the corresponding rename I performed on the GitHub App itself.
We have 2 registered GitHub Apps related to GitHub Actions use cases:

* Nillion Github Actions (already existed)
* And Nillion GitHub Actions Public (new as of today)

The latter is used for public repos and is currently limited to
nada-dsl.

.github/workflows/build.yml

@@ -44,13 +44,13 @@ jobs:
         uses: actions/setup-python@v3
         with:
           python-version: "3.10"
-      - name: Generate Nillion Repo app token
+      - name: Generate GitHub App token
         id: generate-token
         uses: actions/create-github-app-token@v1
         with:
-          app-id: ${{ vars.NILLION_REPO_APP_ID }}
+          app-id: ${{ secrets.NILLION_GITHUB_ACTIONS_PUBLIC_APP_ID }}
           owner: NillionNetwork
-          private-key: ${{ secrets.NILLION_REPO_APP_PRIVATE_KEY }}
+          private-key: ${{ secrets.NILLION_GITHUB_ACTIONS_PUBLIC_APP_PRIVATE_KEY }}
           repositories: nillion
       - name: Checkout tools repo
         uses: actions/checkout@v4