nixpkgs-manual: avoid lib.fileset

I don’t love this, and I’m not convinced it doesn’t have the
same pitfalls as filesets, but see the `flattenReferencesGraph`
commit for reasoning.

It may be better to ban the relevant builtins entirely and just move
things into subdirectories when needed, but I didn’t want to do
more surgery to this part of the tree than necessary to solve the
immediate problem.

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

@@ -90,10 +90,7 @@
 stdenvNoCC.mkDerivation {
   name = "nixpkgs-lib-docs";
 
-  src = lib.fileset.toSource {
-    root = ../..;
-    fileset = ../../lib;
-  };
+  src = ../../lib;
 
   nativeBuildInputs = [
     nixdoc
@@ -101,6 +98,8 @@ stdenvNoCC.mkDerivation {
   ];
 
   installPhase = ''
+    cd ..
+
     export NIX_STATE_DIR=$(mktemp -d)
     nix-instantiate --eval --strict --json ${./lib-function-locations.nix} \
       --arg nixpkgsPath "./." \