Use $CXX env variable when available for determining multiarch

[johnwason]

The function get_multiarch() calls gcc using subprocess. gcc is hard coded, and does not check the CXX env variable. This breaks a lot of build environments, including conda-smithy which uses prefixed gcc executable, and sets CXX to the correct location. This will also break cross compilers that use prefixes.

colcon-ros/colcon_ros/task/cmake/__init__.py

Line 46 in eb0b932

output = subprocess.check_output(['gcc', '-print-multiarch'])

[cottsay]

Using the CXX variable seems reasonable to me, especially in the context of CMake, which also uses that variable.

Please consider opening a PR to change the referenced code to use CXX.