Also pretend novel arch value MINGW6 is really CYGWIN so we'll recognize it.

and use these variants for GitHub builds.

which should only affect Windows unit tests, but should hopefully improve our
chances that Windows unit tests will succeed.

The Python child processes used by llprocess_test.cpp and llleap_test.cpp need
the Python llsd module to communicate with the C++ parent process.

Also set LOGFAIL and BUGSPLAT_DB environment variables.