Shutdown scripts do not work on GCP images

[Ed Maste]

As reported in Cirrus-CI's issue tracker[1] it looks like shutdown
scripts on our GCP images just don't work. This results in spurious
false failures in Cirrus-CI.

Quoting from the report:
> Cirrus CI also relies on shutdown script to detect preeptions: if
> shutdown script is executed before while task is still executing
> that mean that something from the outside killed it e.g. potential
> preemtion.

Where does the shutdown script come from?

[1] https://github.com/cirruslabs/cirrus-ci-docs/issues/592