- Move GRUB installation to occur BEFORE unmounting target filesystem
- Install GRUB inside configure_target_system method before cleanup
- Ensure all mount/umount operations use clean environment variables
- Specifically empty LD_PRELOAD variable to prevent PyInstaller interference
- Update partition_disk, mount_target, and install_bootloader methods
- Remove duplicate GRUB installation call from main run method

This ensures GRUB can access the mounted target filesystem and prevents
PyInstaller environment variables from interfering with system calls.