CI: Run package job only if Github secrets are available

following a trick learned from actions/runner#520

Author: nomeata

.github/workflows/build.yml

@@ -2,6 +2,10 @@ name: "Build and package"
 on:
   pull_request:
   push:
+
+env:
+  SECRETS_ARE_AVAILABLE: ${{ secrets.SECRETS_ARE_AVAILABLE }}
+
 jobs:
   build:
     strategy:
@@ -67,8 +71,8 @@ jobs:
   package:
     # this will only work when the jobs above have uploaded
     # their stuff to the nix cache, which will only work
-    # in this repository
-    if: github.repository == 'entropia/tip-toi-reveng'
+    # if secrets are available
+    if: ${{ env.SECRETS_ARE_AVAILABLE }}
     runs-on: ubuntu-latest
     needs: build
     steps: