execute_process: Restore support for short timeouts

Since commit 5420639a8d (cmExecuteProcessCommand: Replace cmsysProcess
with cmUVProcessChain, 2023-06-01, v3.28.0-rc1~138^2~8) we've
occasionally observed immediate timeouts on processes that take longer
than the timeout to start, even though we only start the timer after the
child processes start.  The problem is that:

* `uv_loop_init` initializes `uv_loop_t`'s cached "now" time.
* Starting processes takes time but does not update the "now" time.
* `uv_timer_start` computes expiry relative the cached "now" time,
  so short timers may be expired as soon as they are started.
* `uv_run` invokes expired timer callbacks before polling for I/O
  or process completion, so we "timeout" immediately.

Fix this by updating the cached "now" time via `uv_update_time` just
before starting timers.  This is needed only for timers that start
before the `uv_run` event loop.  Update our `uv_timer_ptr` wrapper
to make all callers consider the choice when calling `start()`.

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