x/build/env/freebsd-amd64: doesn't handle reboot and/or termination requests

[dmitshur]

A swarming bot may trigger a reboot or shutdown of a bot running in GCE, a process that involves /sbin/shutdown. See relevant Swarming Bot documentation here:

https://chromium.googlesource.com/infra/luci/luci-py/+/master/appengine/swarming/doc/Bot.md

This seems not to work for freebsd/amd64 builders in GCE. For example, a recent bot had the following history of tasks:

Notably, the bot kept going rather than being terminated.

This process seems to work for openbsd/amd64 builders in GCE.

The difference might be that in the openbsd case the swarming user is given access to execute /sbin/shutdown (here), while the freebsd image might not have taken that or some equivalent measure. There might also be more to it.

CC @golang/release, @golang/freebsd.