Move back gRPC_CARES_PROVIDER to CACHE STRING FORCE

Author: raulcd

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -3215,7 +3215,9 @@ function(build_grpc)
   endif()
 
   # Use "none" provider for c-ares or re2, either we vendored it or we already found it.
-  set(gRPC_CARES_PROVIDER "none")
+  set(gRPC_CARES_PROVIDER
+      "none"
+      CACHE STRING "" FORCE)
   set(_gRPC_CARES_LIBRARIES "c-ares::cares")
 
   set(gRPC_RE2_PROVIDER "none")