Make MSYS2 CI jobs actually use MSYS2 (#1081)
Make MSYS2 CI jobs actually use MSYS2 (#1081)

The jobs were executed in powershell using the globally installed cmake.
This makes things actually run in a MSYS2 shell.

This also removes the msys/cygwin job because it doesn't build
(it complains about undeclared posix_memalign)
lemire/simdjson,lemire/simdjson,lemire/simdjson,lemire/simdjson,lemire/simdjson,lemire/simdjson