lib,doc: remove obvious usages of toString on paths

It gives a warning on the lazy-trees branch of Nix
(NixOS/nix#6530)

one of these was also giving me an error (the one in lib/trivial probably)

```
$ nix build
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/installer/sd-card/sd
-image-aarch64.nix' and then accessing it is deprecated, at /home/artturin/nixgits/my-nixpkgs/lib/modules.
nix:349:99
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/.git' and then accessing it is dep
recated, at /home/artturin/nixgits/my-nixpkgs/lib/sources.nix:35:32
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/system/etc/etc.nix'
and then accessing it is deprecated, at «stdin»:0
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/system/etc/etc-activ
ation.nix' and then accessing it is deprecated, at «stdin»:0
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/installer/sd-card/sd
-image-aarch64.nix' and then accessing it is deprecated, at «stdin»:0
error: cannot decode virtual path '/nix/store/virtual0000000000000000000000005-source'
(use '--show-trace' to show detailed location information)
```

1 files changed, 1 insertions, 1 deletions

diff --git a/doc/doc-support/lib-function-locations.nix b/doc/doc-support/lib-function-locations.nix
index 68edd27098543..ac4da88052da1 100644
--- a/doc/doc-support/lib-function-locations.nix
+++ b/doc/doc-support/lib-function-locations.nix
@@ -38,7 +38,7 @@ let
       substr = builtins.substring prefixLen filenameLen filename;
       in substr;
 
-  removeNixpkgs = removeFilenamePrefix (builtins.toString pkgs.path);
+  removeNixpkgs = removeFilenamePrefix pkgs.path;
 
   liblocations =
     builtins.filter