After a helpful suggestion from AppVeyor support staff, moving the Stockfish
execution from ps to cmd seems to work. Alternative to PR #1624 tested in PR #1637.
No functional change.
And set x86 and x64 platforms for real.
Currently this is broken and the same binary is compiled for all platforms.
This is becuase we use a custom build step. OTH the default
build step seems not compatible with cmake generated *sln file.
No functional change.