fix(docs): fixes nix build .#docs

[taliyahwebb]

The Problem

Currently nix build github:rustshop/flakebox#docs and therefore also flakebox docs fails.
With this change the build succeeds nix build github:taliyahwebb/flakebox/docs-fix#docs

Basic Analysis

In my testing i figured out that the stdenv default builder tries to execute the output of the internal optionsDocMd derivation which is added to the docs buildInputs.

From as far as I can tell this only happens when a derivation is created from pkgs.runCommand and writes directly to $out instead of creating a directory and putting a file in that.

Details

I was able to find out that duringsource $stdenv/setup any derivation output that is formed like described above is attempted to be executed by what i assume to be bash.
I was unable to investigate a further cause and didn't find any documentation or mention of this behavior so this might be a nixpkgs.stdenv bug?

Alternative Fix

An alternative fix for the build failure that I attempted was to simply remove optionsDocMd from buildInputs but keep it during build phase, which seemed to work initially.
I am not familiar enough with nix yet to know if that would cause any unwanted side effects and thus decided to make a fixing change to the optionsDocMd derivation instead.

[dpc]

Thanks!