ci: install msys2 toolchain on windows

[tzanko-matev]

Summary

• install MSYS2/MinGW and capnproto on Windows runners
• drop LLVM and vcpkg setup
• add msys64 bin directory to PATH and export clang variables

Testing

• just build-extension
• just test (fails: Content for p mismatch)

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https://chatgpt.com/codex/tasks/task_e_6890b1451acc8323bf286afe8736a918

[github-actions]

Benchmark Report

Benchmark	Ruby (no tracing)	JSON	CAPNP	JSON (PureRuby)
heavy_work	69ms	✓ 465ms 78,110,688B	344ms 204,055B	✓ 36,445ms 211,728,983B

[nickysn]

Some of the test failures look similar to this bug that I've discovered today: #105

[nickysn]

I fixed #105. Can you try to sync with 'main', so we can see whether this resolves the testsuite failures on Windows as well?

[nickysn]

Fixed in a different way:

1. Ruby comes with its own MSYS2 install, so we now use that, instead of installing a different one.
2. Therefore Capnproto and other things are installed using Ruby's MSYS2 package manager pacman.
3. Fixing Rust:
   The official Ruby binaries are compiled with MSYS2 gcc for the UCRT64 environment: https://www.msys2.org/docs/environments/
   Rust by default supports three 64-bit Windows targets:

• x86_64-pc-windows-gnu
• x86_64-pc-windows-msvc
• x86_64-pc-windows-gnullvm
  The default one is x86_64-pc-windows-msvc, which is ABI compatible with MSVC++, but not with the MSYS2 gcc UCRT64, used by Ruby. Therefore we install and use the x86_64-pc-windows-gnu Rust target, using rustup.

4. After the basic compilation works, lots of little fixes were required, since the code for the native recorder has never worked on Windows before. These were mostly directory/path separator issues, file extension issues (e.g. Rust compiles to a file with a .dll extension, but it turned out that Ruby expects the .so extension even on Windows).

Closing this issue.