github_packages: use exponential backoff when retrying

The retry behaviour in `publish_commit_bottles.yml` [1] is often
successful after the second try, so it's likely that we're not waiting
long enough in between retries here.

Let's fix that by retrying with exponential backoff instead of adding a
fixed interval of five seconds after each failure.

[1] 3241035b2a/.github/workflows/publish-commit-bottles.yml (L431-L443)

Library/Homebrew/github_packages.rb

@@ -440,7 +440,7 @@ class GitHubPackages
       rescue ErrorDuringExecution
         retry_count += 1
         odie "Cannot perform an upload to registry after retrying multiple times!" if retry_count >= 5
-        sleep 5*retry_count
+        sleep 5 ** retry_count
         retry
       end