move nccl.h find code to avoid cmake error (#25589)

cmake/nccl.cmake

@@ -7,14 +7,14 @@ if(WIN32)
     return()
 endif()
 
-set(NCCL_ROOT "/usr" CACHE PATH "NCCL ROOT")
-find_path(NCCL_INCLUDE_DIR nccl.h
+if(WITH_NCCL)
+    set(NCCL_ROOT "/usr" CACHE PATH "NCCL ROOT")
+    find_path(NCCL_INCLUDE_DIR nccl.h
         PATHS ${NCCL_ROOT} ${NCCL_ROOT}/include ${NCCL_ROOT}/local/include
         $ENV{NCCL_ROOT} $ENV{NCCL_ROOT}/include $ENV{NCCL_ROOT}/local/include
         NO_DEFAULT_PATH
-)
+    )
 
-if(WITH_NCCL)
     file(READ ${NCCL_INCLUDE_DIR}/nccl.h NCCL_VERSION_FILE_CONTENTS)
 
     string(REGEX MATCH "define NCCL_VERSION_CODE +([0-9]+)"