Prefer Socket.gethostname with fallback to hostname command in hostname method

[Y-Asahi-dev]

This PR updates the hostname method in Resque::Failure::JobAuditLog to:

• Use Socket.gethostname as the primary way to retrieve the server name
• Fallback to the hostname shell command if Socket.gethostname returns empty
• Only memoize successful hostname values and avoid memoizing "(unknown)"

These changes improve reliability in environments where the hostname command may fail
(e.g., restricted PATH, missing binary, or containerized environments) and prevent
permanent "(unknown)" values from being cached in long-lived processes.

No other behavior changes are introduced.