Fix GTest installation during test builds

Force INSTALL_GTEST off before fetching googletest so enabling CDDP_CPP_BUILD_TESTS does not install the bundled dependency alongside cddp.

Author: astomodynamics

CMakeLists.txt

@@ -109,6 +109,7 @@ FetchContent_MakeAvailable(autodiff)
 # Googletest
 if (CDDP_CPP_BUILD_TESTS)
   enable_testing()
+  set(INSTALL_GTEST OFF CACHE BOOL "Don't install GTest alongside cddp" FORCE)
   FetchContent_Declare(
     googletest
     GIT_REPOSITORY https://github.com/google/googletest.git