Add a script to convert the contents of ci/vars.yml to Azure Pipelines
logging commands, which effectively set runtime variables in a pipeline.
We need this script as a workaround for a missing Azure Pipelines
feature: variables are not inherited in extended templates, and reading
the vars.yml file in a "extends" template isn't possible either (at
least not if we want to use the exact revision of the ibex repository
which triggered CI).
