[ci] Don't use babysitter on Windows/cygwin

babysitter generates some temp paths in python when running under cygwin, which breaks down
when those paths are passed to the native Windows mono.exe during testing:

    Exception in TestRunnerThread ---> System.IO.DirectoryNotFoundException: Could not find a part of the path "C:\cygdrive\c\WINDOWS\TEMP\tmpC4K5yK\babysitter_report_current_test_file.txt".