[CI] Create releases from nightly builds

Artifacts from GitHub Actions are only [kept up to 90 days for public repositories](https://docs.github.com/en/organizations/managing-organization-settings/configuring-the-retention-period-for-github-actions-artifacts-and-logs-in-your-organization), so earlier builds are no longer accessible - we should instead generate releases that contain the nightly build.

As a bonus, we can then upload the apk file directly rather than the zipped version