Assume pre-configured environment variables instead of setup script for Connext

[nuclearsandwich]

@clalancette reported these warnings showing up in the Windows builds:

WW> Asked to use Connext but the RTI env was not found at 'C:\Program Files\rti_connext_dds-5.3.1\resource\scripts\rtisetenv_x64Win64VS2017.bat'

The ROS 2 cookbook is setting environment variables used by rti_connext_dds_cmake_module and test_security. The Windows setup should check that those are set rather than overriding them with files found on the host.

[nuclearsandwich]

Rolling Foxy Galactic