Use DELETE_PACKAGES_TOKEN (plural), not DELETE_PACKAGE_TOKEN (#31)

The org secret is named DELETE_PACKAGES_TOKEN. The previous PR
referenced the singular form, which resolved to empty at runtime
and caused the prune step to fail with "token value is not valid".

Author: SeanTAllen

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

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