Added an Ubuntu package mirror for us-east-1 runners
Ubuntu runners in us-east-1 now use Depot’s private mirror for Ubuntu packages and repository metadata.
The mirror is synchronized every six hours from Ubuntu’s official Archive and Ports rsync services. It contains packages for both AMD64 and ARM64 runners.
The mirror is currently available only to runners in us-east-1. Runners in other regions continue to use their existing Ubuntu package sources.