Motivation

This is analogous to that for experimental settings and flags that we have also added as of late.

Context

This would be useful for both the overlay-local store, and also the mounted-ssh:// store (#7912). Indeed I first wrote this commit for the latter and pushed it onto that branch. Now I am pulling it into a separate PR for the former too.

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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