`on-machine` back-end: automating `generate-vars`

[KiaraGrouwstra]

one current limitation to using vars with its provided on-machine back-end is the onus it places on the user to ensure the generate-vars script is triggered.

it would be interesting to perhaps facilitate this. while that perhaps presumes no prompts requiring user input need to be populated still, in contexts without prompt-based generators ensuring relevant variables are generated would seem like a common need in using this back-end.

i would imagine automation here would involve perhaps a one-of service unit triggering the script, possibly using the modular services interface to prevent tight coupling with systemd.

now, in order to make this more robust, this may need a way to skip any prompt-based vars - an 'unattended' flag to the script, basically.

[KiaraGrouwstra]

for now i'm using a naive approach, not handling the prompt case