[BUILD] Fix build without vcpkg on Windows if gRPC is disabled

Currently, if support for gRPC is disabled by configuring the build
with `-DWITH_OTLP_GRPC=OFF`, you are on Windows and do not use
a toolchain file then the build will always use all third-party
dependencies from vcpkg, regardless of whether nlohmann-json, abseil
and protobuf were found or not.

The root cause for this behavior is a bug in the condition meant
to check that all dependencies have been found, which unconditionally
checks for `NOT gRPC_FOUND`, but the corresponding `find_package`
call is only performed if `WITH_OTLP_GRPC` is true, and thus causing
that condition to always evaluate to `true` if `WITH_OTLP_GRPC` is
false.

CMakeLists.txt

@@ -385,7 +385,7 @@ if(WITH_OTLP_GRPC
   if(WITH_OTLP_GRPC)
     find_package(gRPC)
   endif()
-  if((NOT Protobuf_FOUND AND NOT PROTOBUF_FOUND) OR (NOT gRPC_FOUND))
+  if((NOT Protobuf_FOUND AND NOT PROTOBUF_FOUND) OR (WITH_OTLP_GRPC AND NOT gRPC_FOUND))
     if(WIN32 AND (NOT DEFINED CMAKE_TOOLCHAIN_FILE))
       install_windows_deps()
     endif()