nixpkgs-manual.lib-docs: fix sandboxed build on darwin

The darwin sandbox prevents this nix call from accessing `/nix/
store/.links` resulting in a build failure:

```
> error: getting status of '/nix/store/.links': Operation not permitted
```

Thanks to @emilazy for the workaround!

Fixes #339808

doc/doc-support/lib-function-docs.nix

@@ -106,6 +106,7 @@ stdenvNoCC.mkDerivation {
       --arg nixpkgsPath "./." \
       --argstr revision ${nixpkgs.rev or "master"} \
       --argstr libsetsJSON ${lib.escapeShellArg (builtins.toJSON libsets)} \
+      --store $(mktemp -d) \
       > locations.json
 
     function docgen {