fix: make build-gateway-local error (#3302)

[wssaidong]

Ⅰ. Describe what this PR did

修复了 make build-gateway-local 在 ARM64 架构下构建时仍然使用 AMD64 架构的问题。

问题背景：

• 指定了 TARGET_ARCH=arm64 后，构建脚本仍然显示 Architectures: [linux/amd64]
• Makefile 中的 build-golang-filter 目标硬编码了双架构构建
• ./tools/docker 默认使用 --architectures linux/amd64

解决方案：
在 tools/hack/build-istio-image.sh 中添加了 DOCKER_ARCHITECTURES 环境变量的设置逻辑，根据 TARGET_ARCH 动态设置正确的架构：

• TARGET_ARCH=arm64 → DOCKER_ARCHITECTURES=linux/arm64
• 其他情况 → DOCKER_ARCHITECTURES=linux/amd64

并将该变量传递给 Docker 构建流程。

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Ⅱ. Does this pull request fix one issue?

Fixes #3302

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Ⅲ. Why don't you add test cases (unit test/integration test)?

这是一个构建脚本修复，修改的是 Shell 脚本的环境变量设置逻辑。修复可以通过实际执行 make build-gateway-local 命令并验证生成的镜像架构来验证，不需要额外的单元测试或集成测试。

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Ⅳ. Describe how to verify it

验证步骤：

1. 在 ARM64 环境下验证：
   TARGET_ARCH=arm64 make build-gateway-local
2. 预期输出应包含 DOCKER_ARCHITECTURES=linux/arm64
3. 在 AMD64 环境下验证：
   TARGET_ARCH=amd64 make build-gateway-local
4. 预期输出应包含 DOCKER_ARCHITECTURES=linux/amd64
5. 验证生成的镜像架构：
   docker inspect | grep Architecture
6. 应与指定的 TARGET_ARCH 一致

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Ⅴ. Special notes for reviews

• 此修改仅影响 build-gateway-local 目标，不影响其他构建目标
• DOCKER_ARCHITECTURES 变量在原构建脚本中已有使用，这里只是显式设置了它的��
• 默认行为保持不变：未指定 TARGET_ARCH 时仍使用 linux/amd64

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Ⅰ. Describe what this PR did

Fixed issue with make build-gateway-local still using AMD64 architecture when building under ARM64 architecture.

Problem background:

• After specifying TARGET_ARCH=arm64, the build script still shows Architectures: [linux/amd64]
• The build-golang-filter target in the Makefile hardcodes the dual-arch build
• ./tools/docker uses --architectures linux/amd64 by default

Solution:
Added setting logic for the DOCKER_ARCHITECTURES environment variable in tools/hack/build-istio-image.sh to dynamically set the correct architecture based on TARGET_ARCH:

• TARGET_ARCH=arm64 → DOCKER_ARCHITECTURES=linux/arm64
• Other cases → DOCKER_ARCHITECTURES=linux/amd64

and pass this variable to the Docker build process.

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Ⅱ. Does this pull request fix one issue?

Fixes #3302

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Ⅲ. Why don't you add test cases (unit test/integration test)?

This is a build script fix that modifies the environment variable setting logic of the shell script. The fix can be verified by actually executing the make build-gateway-local command and verifying the resulting image architecture, no additional unit tests or integration tests are required.

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Ⅳ. Describe how to verify it

Verification steps:

1. Verify in ARM64 environment:
   TARGET_ARCH=arm64 make build-gateway-local
2. Expected output should contain DOCKER_ARCHITECTURES=linux/arm64
3. Verify in AMD64 environment:
   TARGET_ARCH=amd64 make build-gateway-local
4. Expected output should contain DOCKER_ARCHITECTURES=linux/amd64
5. Verify the generated image architecture:
   docker inspect | grep Architecture
6. Should be consistent with the specified TARGET_ARCH

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Ⅴ. Special notes for reviews

• This modification only affects the build-gateway-local target and does not affect other build targets
• The DOCKER_ARCHITECTURES variable has been used in the original build script, and its value is just explicitly set here.
• Default behavior remains unchanged: linux/amd64 is still used when TARGET_ARCH is not specified

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[CLAassistant]

All committers have signed the CLA.

[CH3CHO]

CC @johnlanni