fix: remove ARG TARGETARCH default in Dockerfile.test-tools

[ycpss91255]

Summary

Critical latent bug shipped since multi-arch matrix landed (v0.9.10+). The arm64 variant of every GHCR test-tools:* image contains x86_64 shellcheck / hadolint binaries, silently breaking downstream arm64 CI at first consumption.

Root cause

Dockerfile.test-tools declares ARG TARGETARCH=amd64 with a default. Per moby/buildkit#3403, a default value shadows BuildKit's per-platform auto-inject. Result: multi-arch builds of this file run the case "${TARGETARCH}" in ... branch with TARGETARCH=amd64 regardless of the actual build platform → x86_64 binary URL used for both amd64 and arm64 manifests.

Confirmation

Anonymous pull of ghcr.io/ycpss91255-docker/test-tools:v0.9.13 arm64 variant:

$ docker create --platform linux/arm64 ghcr.io/.../test-tools:v0.9.13
$ docker cp ...:/usr/local/bin/shellcheck /tmp/sc-arm64
$ file /tmp/sc-arm64
/tmp/sc-arm64: ELF 64-bit LSB executable, x86-64, ...

ros1_bridge PR #27 enabled arm64 matrix and hit:

shellcheck: Exec format error

Fix

-ARG TARGETARCH=amd64
+ARG TARGETARCH

Regression test in template_spec.bats asserts the ARG has no default (^ARG TARGETARCH= must not match) while the bare declaration (^ARG TARGETARCH$) still does. 674 → 675.

Release plan (post-merge)

1. Tag v0.10.0-rc2 from main after this PR merges.
2. release-test-tools.yaml auto-reissues ghcr.io/.../test-tools:v0.10.0-rc2 + :latest with per-arch-correct binaries.
3. Downstream ros1_bridge PR docs: remove i18n for TEST.md and CHANGELOG.md #27 bumps main.yaml @v0.9.13 → @v0.10.0-rc2, arm64 matrix goes green.
4. If validation passes, promote rc2 → v0.10.0.

Impact on existing tags

v0.9.13, v0.10.0-rc1 GHCR images stay broken on arm64. Repos currently pinned to those tags that only build amd64 are unaffected. Repos with arm64 matrix enabled must move to @v0.10.0-rc2+ — or the template bumps its minimum supported tag if we want to deprecate the broken ones.

Test plan

• make -f Makefile.ci test local — 675/675 pass
• Self-test CI green on this PR
• Post-tag: inspect arm64 variant of :v0.10.0-rc2, confirm shellcheck is aarch64:

  docker create --platform linux/arm64 ghcr.io/.../test-tools:v0.10.0-rc2
  docker cp ...:/usr/local/bin/shellcheck /tmp/sc-arm64
  file /tmp/sc-arm64   # expect: aarch64
  

• Post-tag: ros1_bridge PR docs: remove i18n for TEST.md and CHANGELOG.md #27 @v0.10.0-rc2, arm64 CI shard green.