fix: correct $VERSION reference in build-iso.sh, fix QEMU networking

- src/build-iso.sh: Replace undefined $VERSION with correct filename.
  The success check referenced $PROJECT_NAME-v$VERSION.iso but $VERSION
  was never defined, causing the build to always report failure.
- scripts/validate-iso.sh: Replace deprecated `-net nic -net user` with
  modern `-nic user` QEMU networking syntax.

💘 Generated with Crush

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

scripts/validate-iso.sh

@@ -226,7 +226,7 @@ phase2_boot_test() {
         -drive file="$VM_DISK",format=qcow2,if=virtio \
         -cdrom "$ISO_PATH" \
         -boot d \
-        -net nic -net user \
+        -nic user \
         -serial file:"$SERIAL_LOG" \
         -display none \
         -no-reboot \