home-manager: Add home-manager search path to EXTRA_NIX_PATH

[zhaofengli]

Description

Since 2.4, the Nix installer no longer adds the NIX_PATH environment variable to shell profiles, because Nix now has a built-in search path if it's unset. However, home-manager still relies on it being set and prepends search paths for home-manager. Even though the documentations have been updated to include instructions for non-NixOS users, this is a bad first-time experience (https://github.com/nix-community/home-manager/issues/2564, https://github.com/nix-community/home-manager/issues/3020, https://github.com/nix-community/home-manager/issues/2580).

Instead of relying on NIX_PATH being set and prepending to it, let's not touch it at all and add our paths as -I arguments. We trust that Nix will use a sensible default.

Tested by installing home-manager on Ubuntu 22.04 with a fresh multi-user Nix installation.

Checklist

• [x] Change is backwards compatible.

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