[CI][C++] MinGW jobs fail every run after MSYS2 toolchain updates

[tadeja]

Describe the bug, including details regarding any error messages, version, and platform.

The MINGW64 and CLANG64 MinGW jobs in .github/workflows/cpp.yml have failed every run since 2026 April 30 - first failure on main: https://github.com/apache/arrow/actions/runs/25213414021/job/73928546151#step:12:2644

The following tests FAILED:
	 41 - arrow-async-utility-test (Exit code 0xc0000374)   arrow-tests unittest
	 62 - arrow-dataset-dataset-writer-test (SEGFAULT)      arrow_dataset unittest
	 65 - arrow-dataset-file-test (Failed)                  arrow_dataset unittest
Error: Process completed with exit code 8.

Recent failure 2026 May 05 - https://github.com/apache/arrow/actions/runs/25379694834/job/74424280352#step:12:2468

The following tests FAILED:
	 41 - arrow-async-utility-test (Exit code 0xc0000374)   arrow-tests unittest
	 44 - arrow-threading-utility-test (Timeout)            arrow-tests unittest
	 62 - arrow-dataset-dataset-writer-test (Failed)        arrow_dataset unittest
	 65 - arrow-dataset-file-test (Failed)                  arrow_dataset unittest
	 76 - arrow-s3fs-test (Failed)                          arrow-tests filesystem unittest
Error: Process completed with exit code 8.

Likely triggered by two MSYS2 updates:

1. MINGW64: gcc 15.2 -> 16.1 probably made the thread_local race in ThreadPool ([C++][CI] arrow-json-test segfaults occasionally on AMD64 Windows MinGW MINGW64 C++ job #49272) crash now on every run. Expected fix: GH-49272: [C++][CI] Fix intermittent segfault in arrow-json-test with MinGW #49462.
2. MINGW64 + CLANG64: aws-sdk-cpp 1.11.479 -> 1.11.801 stopped sending Content-Md5 on DeleteObjects; the bundled MinIO (RELEASE.2024-09-13) still requires it. Fix: bump bundled MinIO, or configure the SDK to keep sending MD5 (?).

Draft PR with a very temporary workaround (pin the packages to last known good versions) : #49931

This issue tracks reverting to current upstream MSYS2 packages once

• GH-49272: [C++][CI] Fix intermittent segfault in arrow-json-test with MinGW #49462 is merged and
• the aws-sdk-cpp / MinIO incompatibility is resolved.

Component(s)

Continuous Integration, C++