Function acquire_lock 

pub fn acquire_lock() -> Result<LockGuard, Error>

Acquires a system-wide lock to prevent concurrent modifications to the Zoi store.

Locking Mechanism:

• Attempts to open/create ~/.zoi/pkgs/lock.
• Uses flock (via fs2) to acquire an exclusive advisory lock on the file.
• If busy, reads the file to display the PID of the process currently holding the lock.
• Once acquired, writes the current process PID to the lock file.

This ensures that operations like install, uninstall, and update never run simultaneously, preventing database and filesystem corruption.