src: fix spawnSync CHECK when SIGKILL fails

We might not have sufficient privileges to signal the child process
so don't make assumptions about the return value of `uv_process_kill()`.

Example:

    node -e 'child_process.spawnSync("sudo", ["ls"], { maxBuffer: 1 })'

No test because:

1. The test needs to run as root (can't invoke sudo), and

2. The parent needs to drop privileges but can't, because
   then the child process won't have sufficient privileges.

Fixes: nodejs#31747

src/spawn_sync.cc

@@ -607,8 +607,9 @@ void SyncProcessRunner::Kill() {
     if (r < 0 && r != UV_ESRCH) {
       SetError(r);
 
-      r = uv_process_kill(&uv_process_, SIGKILL);
-      CHECK(r >= 0 || r == UV_ESRCH);
+      // Deliberately ignore the return value, we might not have
+      // sufficient privileges to signal the child process.
+      USE(uv_process_kill(&uv_process_, SIGKILL));
     }
   }