`om develop`: Support `attic`

`attic use` should be run in the `preShell` if an attic cache is configured in `om.health.caches.required`, like the existing behaviour when a `cachix` cache is configured.

There is a challenge here: We cannot auto-detect `attic` caches like we do for `cachix` from the domain name (`.cachix.org`).

Start at the `om develop` entry point and inspect how `preShell` handles configured Cachix caches from `om.health.caches.required`. Trace how cache entries are represented and determine how an Attic cache can be distinguished without Cachix domain detection. Done means `attic use` runs for a configured Attic cache, with coverage for that configuration.

Written by the indexing model from the issue text.