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/programs/environment.nix

@@ -30,7 +30,7 @@ in
       ];
 
     # TODO: move most of these elsewhere
-    environment.profileRelativeEnvVars =
+    environment.profileRelativeSessionVariables =
       { PATH = [ "/bin" ];
         INFOPATH = [ "/info" "/share/info" ];
         KDEDIRS = [ "" ];