The previous cleanup logic sorted releases by column 3 which contained
relative dates like '3 days ago' as text. This doesn't work reliably
for sorting because '3 days ago' and '4 days ago' are compared as
strings, not dates.
This fix:
- Uses gh release list --json to get actual ISO 8601 creation timestamps
- Calculates a proper cutoff date (3 days ago in seconds since epoch)
- Parses and compares actual timestamps
- Only deletes releases that are truly older than 3 days
Fixes the nightly build retention policy.