Utils: only use msys2 paths from msys2 python

This patch attempts to detects if, when we are running from within an MSYS2 environement (MSYSTEM is set) we are also executing inside an MSYS2 provided version of python. It does this by assuming that if we are not in a cygwin environment and we are building on windows, If the value of sys.executable is /usr/bin or /bin or /usr/local/bin (somethign unixy) then we are running in an MSYS2 python interpreter and shoudl compensate for msys2 root paths. Otherwise we shouldn't be doing extra path manipulation.

#2226