cmake : fix ccache conflict (#12522)

If users already set CMAKE_C_COMPILER_LAUNCHER globally, setting it in
cmake again will lead to conflict and compile fail.

Signed-off-by: Jay <BusyJay@users.noreply.github.com>

ggml/src/CMakeLists.txt

@@ -65,7 +65,7 @@ if (GGML_LTO)
     endif()
 endif()
 
-if (GGML_CCACHE)
+if (GGML_CCACHE AND NOT CMAKE_C_COMPILER_LAUNCHER AND NOT CMAKE_CXX_COMPILER_LAUNCHER)
     find_program(GGML_CCACHE_FOUND ccache)
     find_program(GGML_SCCACHE_FOUND sccache)