[build] Generate last-update in GN from jiri history, not from jiri hook

The time stamp of the last `jiri update` is visible in the
`//.jiri_root/update/update_history/` directory.  So rather than
touching the source tree in a hook that runs at the end of `jiri
update`, just generate the timestamp file in the GN build as a
function of the files `jiri update` writes.

Bug: BLD-200 #comment Stop writing //garnet/last-update file in jiri hook
Test: CQ
Change-Id: I41c1448b0fed8d0cb3985ee032d1b7d4c6209a36