Use PACKER_GITHUB_API_TOKEN in environment

Seems like the secret must be set explicitly in the environment for it
to be available to the packer command.

.github/workflows/build.yml

@@ -10,6 +10,7 @@ jobs:
     env:
       PACKER_LOG: 0
       PACKER_CACHE: .cache/packer
+      PACKER_GITHUB_API_TOKEN: ${{ secrets.PACKER_GITHUB_API_TOKEN }}
 
     steps:
       - name: Checkout repository