Fix omarchy-vm boot helper path

[gosuwachu]

Summary

• resolve omarchy-iso-boot relative to the omarchy-vm script directory

Before the fix:

> ./omarchy-vm boot                                                  
Copying VM 'test' to /tmp/omarchy-iso-boot.qcow2...
✓ VM ready. Booting...
./omarchy-vm: line 129: exec: omarchy-iso-boot: not found

Testing

• bash -n bin/omarchy-vm
• bin/omarchy-vm boot should work

[Chessing234]

calls sibling omarchy-iso-boot via script_dir — fixes path-dependent boot failure.