Fix AppVeyor build for Cygwin

[embray]

The AppVeyor build has still been running for a while, but not reporting back to GitHub since the change to GitHub "Apps".

I have re-enabled it, but it appears to be failing when trying to run Python in Cygwin with an unhelpful:

if ( "$Env:OS" -ieq "Cygwin" ) {
    $python = "python" + $Env:PYTHON_VERSION[0] + $Env:PYTHON_VERSION[2]
    Start-Process -NoNewWindow -Wait `
        -FilePath $Env:CYG_ROOT\setup-$Env:ARCH.exe `
        -ArgumentList "-q -P $python,$python-devel,$python-pip,gcc-core,gcc-g++,libcrypt-devel"
}
cd C:\projects\cysignals
python%PYTHON_VERSION% -c "import sys; print(sys.version); print(sys.platform); print(sys.path)"
Access is denied.

It used to work but that was a long time ago to be sure...