Build instructions are wrong

[alyssais]

Problem

The build instructions say to set dontAddPrefix=1 so that the shell try to install into the store, but it still tries to do this for every directory variable except for prefix, because dontAddPrefix in mesonConfigurePhase only affects the prefix, not e.g. the libdir.

Proposal

Checklist

• [x] checked latest Nix manual (source)
• [x] checked open documentation issues and pull requests for possible duplicates

Priorities

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