workflows/labels: prevent error on token creation for Test workflow

This only happens when the label workflow runs in pull_request context
*and* from within nixpkgs (not a fork). This is the case for dependabot
updates.

.github/workflows/labels.yml

@@ -50,7 +50,7 @@ jobs:
 
       # Use a GitHub App, because it has much higher rate limits: 12,500 instead of 5,000 req / hour.
       - uses: actions/create-github-app-token@0f859bf9e69e887678d5bbfbee594437cb440ffe # v2.1.0
-        if: vars.NIXPKGS_CI_APP_ID
+        if: github.event_name != 'pull_request' && vars.NIXPKGS_CI_APP_ID
         id: app-token
         with:
           app-id: ${{ vars.NIXPKGS_CI_APP_ID }}