add the `buildInputs` man pages to MANPATH

[Profpatsch]

https://github.com/NixOS/nix/pull/4702 is being blocked by Eelco, but having manpages available in your project environments is a really good thing.

We should make an effort to add them into the lorri environment.

[sternenseemann]

There are more or less three approaches to this left which seem to be the preferred outcome for upstream nix (since they don't involve upstream nix changing anything, but they won't work for the nixUnstable CLI):

• Add some specific lorri specific workaround for this: meh
• Make mkShell populate MANPATH: while this seems sensible, it'll likely not fix the problem for a lot of development environments which aren't based on mkShell
• Add a setup hook to stdenv which sets up MANPATH: This would solve the problem in lorri and nix-shell completely. However, I'm still convinced that this is a terrible hack since it is completely orthogonal to the purpose of stdenv — nobody needs MANPATH in a build environment

[Profpatsch]

• Add some specific lorri specific workaround for this: meh

• Make mkShell populate MANPATH: while this seems sensible, it'll likely not fix the problem for a lot of development environments which aren't based on mkShell

Eventuall I’d like a dev-shell DSL that we recommend people use, which lorri supports. But for now I think we need to work with what we have.