Fix GCP attack technique failing to impersonate service accounts (clo…

[christophetd]

…ses #299)

What does this PR do?

Motivation

Checklist

• The attack technique emulates a single attack step, not a full attack chain
• We have factual evidence & references that the attack technique was used by real malware, pentesters, or attackers
• The attack technique makes no assumption about the state of the environment prior to warming it up