Create `install.sh` to set environment variables automatically if they haven't been set

We have a `cdds_system_information` script for checking if any environment variables are unset.

If i understand correctly we could do with extending this functionality so it also sets them if they're unset.

This could either be a new `install.sh` script, or possibly be added to `cdds_system_information`.

Start by reading the existing cdds_system_information script to identify which environment variables it checks and how it reports unset values. Compare the two proposed entry points, a new install.sh or an extension to that script. Done means unset variables are set automatically while already-set values remain unchanged, with the existing checking behavior preserved.

Written by the indexing model from the issue text.