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/config/shells-environment.nix

@@ -162,8 +162,17 @@ in
         /bin/sh
       '';
 
+    # For resetting environment with `. /etc/set-environment` when needed
+    # and discoverability (see motivation of #30418).
+    environment.etc."set-environment".source = config.system.build.setEnvironment;
+
     system.build.setEnvironment = pkgs.writeText "set-environment"
        ''
+         # DO NOT EDIT -- this file has been generated automatically.
+
+         # Prevent this file from being sourced by child shells.
+         export __NIXOS_SET_ENVIRONMENT_DONE=1
+
          ${exportedEnvVars}
 
          ${cfg.extraInit}