BUG: Not able to build images (SCION - setup tutorial)

## Description

As mentioned in https://googlecloudplatform.github.io/scion/advanced-local/custom-images/#option-2-local-podman-build, ran commands to build local images.

`image-build/scripts/build-images.sh: line 272: build_arg_flags[@]: unbound variable`

## Steps to Reproduce

Clone the repo, build SCION from it. Run below command from the same local copy

1. scion init
2. image-build/scripts/build-images.sh --builder local-podman --target all

## Expected Behavior

NA

## Actual Behavior

```
$ image-build/scripts/build-images.sh
--builder local-podman --target all
Builder: local-podman (mode: per-image)
Target: all
Registry:
Tag: latest (+ :9d39XXXXX)
Platforms:
Push: false
Steps: core-base scion-base scion-claude scion-gemini
scion-opencode scion-codex scion-hub

image-build/scripts/build-images.sh: line 272: build_arg_flags[@]:
unbound variable
```

## Debug Output

NA

## Environment

- **Scion version**: Commit: 9d39b30d Build Time: 2026-05-14T17:24:13Z
- **OS/Platform**: Mac
- **Runtime**: Podman
- **Harness**: Gemini

Start with image-build/scripts/build-images.sh, especially line 272 and the handling of build_arg_flags[@]. Reproduce with scion init followed by image-build/scripts/build-images.sh --builder local-podman --target all on Mac with Podman. Done means the command completes without the unbound-variable error and the local images build as expected.

Written by the indexing model from the issue text.