Pass DELETE_PACKAGE_TOKEN to container-retention-policy (#30)

GitHub's new installation-token format (longer ghs_ JWT with
hyphens) fails snok/container-retention-policy's hardcoded token
regex; the action is unmaintained at v3.0.1. Use the org-wide
DELETE_PACKAGE_TOKEN classic PAT, whose format isn't changing.

Author: SeanTAllen

.github/workflows/update-nightly-image.yml

@@ -57,7 +57,7 @@ jobs:
         uses: snok/container-retention-policy@3b0972b2276b171b212f8c4efbca59ebba26eceb
         with:
           account: ponylang
-          token: ${{ secrets.GITHUB_TOKEN }}
+          token: ${{ secrets.DELETE_PACKAGE_TOKEN }}
           image-names: library-documentation-action-v2
           tag-selection: untagged
           cut-off: 1d