sched/critmon: Fix CPU load stats incorrect when using SCHED_CPULOAD_CRITMONITOR

Fix the abnormal CPU usage statistics issue caused by missing update
of run_start timestamp in the target task (to) TCB when the CPU load
counting mode is SCHED_CPULOAD_CRITMONITOR.

Before this fix, the to->run_start was not set when switching context,
leading to incorrect CPU usage calculation (e.g., Idle_Task showed 52.9%
CPU usage instead of 100%, and running tasks had wrong non-zero values).
After fixing, the CPU statistics return to normal: Idle_Task correctly
shows 100% usage, and non-running tasks show 0% as expected.

Enable:
+CONFIG_SCHED_CPULOAD_CRITMONITOR=y
+CONFIG_SCHED_CRITMONITOR=y

Before:
nsh> ps
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK    CPU COMMAND
    0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0069584 52.9% Idle_Task
    1     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067456  0.0% sim_loop_wq 0x7b4417a003f0 0x7b4417a00470
    2     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067464 17.6% hpwork 0x4014dba0 0x4014dc20
    3     3     0 100 FIFO     Task      - Running            0000000000000000 0067496 36.8% nsh_main

After:
nsh> ps
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK    CPU COMMAND
    0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0069584 100.0% Idle_Task
    1     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067456  0.0% sim_loop_wq 0x7646932003f0 0x764693200470
    2     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067464  0.0% hpwork 0x4014dba0 0x4014dc20
    3     3     0 100 FIFO     Task      - Running            0000000000000000 0067496  0.0% nsh_main

This issue was introduced by PR #17075, where the run_start update for the
target task was omitted in the SCHED_CPULOAD_CRITMONITOR branch.

Signed-off-by: chao an <anchao.archer@bytedance.com>

sched/sched/sched_critmonitor.c

@@ -375,6 +375,7 @@ void nxsched_switch_critmon(FAR struct tcb_s *from, FAR struct tcb_s *to)
 #ifdef CONFIG_SCHED_CPULOAD_CRITMONITOR
   clock_t tick = elapsed * CLOCKS_PER_SEC / perf_getfreq();
   nxsched_critmon_cpuload(from, current, tick);
+  to->run_start = current;
 #endif
 
 #if CONFIG_SCHED_CRITMONITOR_MAXTIME_THREAD >= 0