treewide: Remove uses of builtins.toPath.

toPath has confusing semantics and is never necessary; it can always
either just be omitted or replaced by pre-concatenating `/.`. It has
been marked as "!!! obsolete?" for more than 10 years in a C++
comment, hopefully removing it will let us properly deprecate and,
eventually, remove it.

nixos/lib/eval-config.nix

@@ -28,7 +28,7 @@
 
 let extraArgs_ = extraArgs; pkgs_ = pkgs;
     extraModules = let e = builtins.getEnv "NIXOS_EXTRA_MODULE_PATH";
-                   in if e == "" then [] else [(import (builtins.toPath e))];
+                   in if e == "" then [] else [(import e)];
 in
 
 let