context : restore preemptive sched reset when LLAMA_SET_ROWS=0 (#14870)

ggml-ci

src/llama-context.h

@@ -287,6 +287,10 @@ private:
 
     bool has_evaluated_once = false;
 
+    // env: LLAMA_SET_ROWS (temporary)
+    // ref: https://github.com/ggml-org/llama.cpp/pull/14285
+    bool supports_set_rows = false;
+
     // perf
     mutable int64_t t_start_us  = 0;
     mutable int64_t t_load_us   = 0;