packaging/everything.nix: Fix doc and man outputs

We want the $doc, $man outputs to be symlinks pointing to nix-manual and
nix-manual.man. Creating the directories first makes the `ln` command
produce symlink $doc/${nix-manual} instead.

```
$file /nix/store/q4dwlnd36gpfajgfcp6hca2xwy068wjq-nix-2.27.1-man/rwh8ky3k040wyrywl8k2v5b3csdfbdg7-nix-manual-2.27.1-man

/nix/store/q4dwlnd36gpfajgfcp6hca2xwy068wjq-nix-2.27.1-man/rwh8ky3k040wyrywl8k2v5b3csdfbdg7-nix-manual-2.27.1-man:
  symbolic link to /nix/store/rwh8ky3k040wyrywl8k2v5b3csdfbdg7-nix-manual-2.27.1-man
```

This is the reason `nix-env --help` is once again broken on 2.26/2.27/master
after 4108529.

Author: xokdvium

packaging/everything.nix

@@ -192,7 +192,7 @@ stdenv.mkDerivation (finalAttrs: {
       devPaths = lib.mapAttrsToList (_k: lib.getDev) finalAttrs.finalPackage.libs;
     in
     ''
-      mkdir -p $out $dev $doc $man
+      mkdir -p $out $dev
 
       # Merged outputs
       lndir ${nix-cli} $out