ci: split S3/MinIO into a Linux and macOS-only workflow

[zhjwpku]

Closes #640

[Copilot]

Pull request overview

This PR addresses CI failures on Windows runners caused by the lack of a Windows-compatible minio/minio Docker image by decoupling S3/MinIO-backed testing from the main cross-platform workflow and running it only on Linux/macOS.

Changes:

• Made S3 support in the CI build script configurable (defaulting to OFF) so non-S3 workflows don’t build/run S3 tests.
• Removed MinIO startup and S3 environment variables from the main test.yml workflow (including Windows).
• Added a dedicated s3_test.yml workflow that runs MinIO-backed S3 tests on Ubuntu and macOS only.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
ci/scripts/build_iceberg.sh	Adds an explicit ICEBERG_S3 ON/OFF switch via a new positional argument so CI can disable S3 where unsupported.
.github/workflows/test.yml	Removes MinIO/S3 env and startup from the main test workflow to avoid Windows Docker manifest issues.
.github/workflows/s3_test.yml	Introduces a separate Linux/macOS workflow to run S3 integration tests against MinIO.

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[wgtmac]

Currently the new split workflows still do the same thing. In the future we may use a dedicated script to build and run components relevant to AWS only.