Environment variables to always set at login.

           The values may refer to other environment variables using POSIX.2 style variable references. For example, a variable parameter may be referenced as $parameter or ${parameter}. A default value foo may be given as per ${parameter:-foo} and, similarly, an alternate
           value bar can be given as per ${parameter:+bar}.

           Note, these variables may be set in any order so no session variable may have a runtime dependency on another session variable. In particular code like

               home.sessionVariables = {
                 FOO = "Hello";
                 BAR = "$FOO World!";
               };

           may not work as expected. If you need to reference another session variable, then do so inside Nix instead. The above example then becomes

               home.sessionVariables = {
                 FOO = "Hello";
                 BAR = "${config.home.sessionVariables.FOO} World!";
               };