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 b00a3fc6
(the original #30418).