Add support for CTest ENVIRONMENT keyword

CTest tests allow to specify ENVIRONMENT variables per test case. Documentation for this feature is here: https://cmake.org/cmake/help/latest/prop_test/ENVIRONMENT.html
This individual environment variable configurations (which are super handy when having integration tests that need special setups) are luckily available via ctest -N --show-only=json-v1, which is the call cmakebuildsystem.cpp uses for selecting all tests.
I want to suggest to add these ENVIRONMENT values to the execution of the individual test cases, appending the currently configured execution environment.