nixos/system-environment: introduce environment.profileRelativeSessionVariables

There is a need for having sessionVariables set relative to the Nix Profiles.
Such as in #68383.

nixos/modules/config/shells-environment.nix

@@ -157,6 +157,8 @@ in
     # terminal instead of logging out of X11).
     environment.variables = config.environment.sessionVariables;
 
+    environment.profileRelativeEnvVars = config.environment.profileRelativeSessionVariables;
+
     environment.shellAliases = mapAttrs (name: mkDefault) {
       ls = "ls --color=tty";
       ll = "ls -l";