v0.2.99-20220317: build: fix getting cmake_cxx_compiler from cache

CMakeCache.txt is showing CMAKE_CXX_COMPILER as a STRING instead of
FILEPATH. When plugin retrieves cache information, it is failing to
recover CMAKE_CXX_COMPILER, and triggering and execution during build.
Searching for STRING if FILEPATH is not found solves the issue.

Signed-off-by: Marta Navarro <[email protected]>