Don't write Python bytecode when invoking pytest (#533)

This should prevent pytest invocations via ament_add_pytest_test from
writing __pycache__ directories into the package sources.

Signed-off-by: Scott K Logan <logans@cottsay.net>

ament_cmake_pytest/cmake/ament_add_pytest_test.cmake

@@ -99,6 +99,8 @@ function(ament_add_pytest_test testname path)
     "--junit-prefix=${PROJECT_NAME}"
   )
 
+  set(ARG_ENV PYTHONDONTWRITEBYTECODE=1 ${ARG_ENV})
+
   if(ARG_NOCAPTURE)
     # disable output capturing
     list(APPEND cmd "-s")