environment.sessionVariables cannot refer to the values of env vars,
and as a result this has caused problems in a variety of scenarios.

One use for these is that they're injected into /etc/profile,
elewhere these are used to populate an 'envfile' for pam
(`pam 5 pam_env.conf`) which mentions use of HOME being
potentially problematic.

Anyway if the goal is to make things easier for users,
simply do the NIX_PATH modification as extraInit.

This fixes the annoying problems generated by the current approach
(NixOS#40165 and others) while hopefully serving the original goal.

One way to check if things are borked is to try:

$ sudo env | grep NIX_PATH

Which (before this change) prints NIX_PATH variable with
an unexpanded $HOME in the value.

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This does mean the following won't contain user channels for 'will':
$ sudo -u will nix-instantiate --eval -E builtins.nixPath

However AFAICT currently they won't be present either,
due to unescaped $HOME.  Unsure if similar situation for other users
of sessionVariables (not sudo) work with current situation
(if they exist they will regress after this change AFAIK).

Per reviewer comment (thanks!).