Environment variables can also be set using your configuration.nix file. This is done using the environment.variables setting:

environment.variables = {
    # Environment variables go here
};

Environment variables with a single value can be defined using a simple assignment of the environment variable name and its corresponding value:

environment.variables = {
    XDG_CONFIG_HOME = "$HOME/.config";
};

Environment variables that use a list of values (e.g. $PATH, $XCURSOR_PATH) can be declared using a Nix Expression list type as shown:

environment.variables = {
    XCURSOR_PATH = [
        "$HOME/.icons"
        "$HOME/.nix-profile/share/icons"
    ];
};

To use the current system path which will be generated when building the NixOS configuration, you can use the Nix Expression string with enclosed Nix expressions:

environment.variables = {
    XCURSOR_PATH = [
        "${config.system.path}/share/icons"
    ];
};

This will map ${config.system.path} to the directory /run/current-system/sw/ when the system is built.