core: use write() to send handoff timestamp

core: use write() to send handoff timestamp

Before 12001b1bf067339db089d52e08fd0b4c6a9945df, the timestamp is sent
with write(), but the commit made the timestamp sent by send(), and
causes regressin #33299.

Note the invocation will still fail if write() is filtered by seccomp.
But, that is an old issue since Type=exec is introduced
(5686391b006ee82d8a4559067ad9818e3e631247).

Partially fixes a regression caused by 12001b1bf067339db089d52e08fd0b4c6a9945df.
Partially fixes #33299.