Currently, we build `man-cache` with `runCommandLocal`, which causes it
to get re-built locally instead of fetched from cache. While the
resulting derivation might be small, it does take a fair bit of time to
build for all my systems, and would be _far_ quicker to fetch.
With this change, we use `runCommand` instead of `runCommandLocal`,
allowing it to get fetched from cache instead of rebuilt for all hosts.
conversions were done using https://github.com/pennae/nix-doc-munge
using (probably) rev f34e145 running
nix-doc-munge nixos/**/*.nix
nix-doc-munge --import nixos/**/*.nix
the tool ensures that only changes that could affect the generated
manual *but don't* are committed, other changes require manual review
and are discarded.
It may be possible to revert the move of `documentation.man.manualPages`
later. The problem is that other man implementations (mandoc) want to
generate their index databases in place, so the approach taken here
doesn't translate super well.
