Fix s3_plugin_e2e: dl.min.io no longer serves MinIO binaries

[tuhaihe]

MinIO archived the open-source Server and Client (mc) and stopped serving their binaries: every artifact under dl.min.io now returns HTTP 410 Gone. The s3_plugin_e2e test case downloads both, so it fails on the first curl.

CI has not run since 2026-09-09, before the shutdown, so this has not gone red yet -- it breaks on the next run of that test case.

Pin the final open-source releases from their GitHub release assets, which are still published, and verify each binary against the sha256sum MinIO publishes next to it. The previous code chmod +x'd an unverified download.

Server RELEASE.2025-09-07T16-13-09Z, client RELEASE.2025-08-13T08-35-41Z. Both keep the minio/mc file names and the default minioadmin credentials, so the server startup, health probe and bucket setup are unchanged.

The release/checksum values are plain shell variables rather than exported env: anything with a MINIO_ prefix would be read as configuration by the MinIO server started a few lines below.

closes: #ISSUE_Number

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[tuhaihe]

Similar to apache/cloudberry#1996

[leborchuk]

LGTM