Revert "nix: put builtin in scope when needed"

I don't know where I got this anti-pattern. It makes the LSP unable to
figure out which bindings are unbound

nix/home/_/tmux/default.nix

@@ -1,5 +1,4 @@
 { pkgs, ... }:
-with builtins;
 {
   programs.tmux = {
     enable = true;
@@ -8,6 +7,6 @@ with builtins;
         set -g default-command "${pkgs.fish}/bin/fish" # Use fish
         set -g default-shell "${pkgs.fish}/bin/fish"
       ''
-      + readFile ./tmux.conf;
+      + builtins.readFile ./tmux.conf;
   };
 }