Hi. In case it helps, debhelper has already a provision for these kind of things:
HOME, XDG_* In compat 13 and later, these environment variables are reset before invoking the upstream build system via the dh_auto_* helpers. The variables HOME (all dh_auto_* helpers) and XDG_RUNTIME_DIR (dh_auto_test only) will be set to a writable directory. All remaining variables and XDG_RUNTIME_DIR (except for during dh_auto_test) will be cleared. The HOME directory will be created as an empty directory but it will be reused between calls to dh_auto_*. Any content will persist until explicitly deleted or dh_clean. So, maybe by using this feature it would be possible to simplify whatever solution you had in mind. (One of the times where a new imagemagick broke a bunch of packages, I remember that this feature, for packages already using compat 13, made the fix to be a lot simpler and nicer). Thanks.