mandoc: allow makewhatis(8) to index the nix store

To make makewhatis(8) index manual pages which are symlinks to the nix
store, we need to set READ_ALLOWED_PATH to include
`builtins.storeDir`. For background and discussion see:
https://inbox.vuxu.org/mandoc-tech/c9932669-e9d4-1454-8708-7c8e36967e8e@systemli.org/T/

This is a small change required as a prerequisite for #139333 which
causes a mass-rebuild for darwin, so it has to go through staging.
Since the change is trivial and has basically no chance of breaking
anything, we'll let it go through staging on its own, the modules
can be reviewed and merged into master later.

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