The makefiles look for python3 on Unix-like systems where python is often
Python 2. This uses sh code so it doesn't work on Windows. On Windows, the
makefiles just assume that python is Python 3.
The code was incorrectly deciding not to try python3 based on WINDOWS_BUILD,
which indicates that the build is *for* Windows. Switch to checking WINDOWS,
which indicates that the build is *on* Windows.
Fix#4774
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
