nixos/shells: avoid overriding the environment for child shells

A shared exported guard `__NIXOS_SET_ENVIRONMENT_DONE` is introduced that can
be used to prevent child shells from sourcing `system.build.setEnvironment`
the second time.

This fixes e.g. `nix run derivation` when run from e.g. ZSH through the console or
ssh. Before this Bash would resource the common environment resetting the `PATH`
environment variable.

We also export `system.build.setEnvironment` to `/etc/set-environment` making it
easy to reset the common environment with `. /etc/set-environment` when
needed and to grep for environment variables in `/etc` (which was the
motivation of #30418).

This reverts changes made in b00a3fc6fd
(the original #30418).

nixos/modules/programs/fish.nix

@@ -109,7 +109,9 @@ in
       set fish_function_path ${pkgs.fish-foreign-env}/share/fish-foreign-env/functions $__fish_datadir/functions
       
       # source the NixOS environment config
-      fenv source ${config.system.build.setEnvironment}
+      if [ -z "$__NIXOS_SET_ENVIRONMENT_DONE" ]
+          fenv source ${config.system.build.setEnvironment}
+      end
 
       # clear fish_function_path so that it will be correctly set when we return to $__fish_datadir/config.fish
       set -e fish_function_path