setup-windows: Msys2 step - MSYS2 with MinGW-w64 GCC, per-user, no elevation

setup-windows: Msys2 step - MSYS2 with MinGW-w64 GCC, per-user, no elevation

Unpacks the MSYS2 base self-extracting archive into %LOCALAPPDATA%\msys64
(the winget package would run the installer machine-wide into C:\msys64),
updates it, and installs GCC for ucrt64 (x86_64) and mingw32 (i686,
msvcrt - the environment that can still target Windows XP). Same
toolchains as GitHub's windows-latest runner. Nothing goes on the PATH;
projects pick an environment's bin directory by full path. x64-only.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>