Rename environment.systemVariables -> environment.sessionVariables

This makes it clearer that they're part of PAM sessions.

nixos/modules/config/i18n.nix

···
 
     environment.systemPackages = [ glibcLocales ];
 
-    environment.systemVariables =
+    environment.sessionVariables =
       { LANG = config.i18n.defaultLocale;
         LOCALE_ARCHIVE = "/run/current-system/sw/lib/locale/locale-archive";
       };

nixos/modules/config/system-environment.nix

···
 
   options = {
 
-    environment.systemVariables = mkOption {
+    environment.sessionVariables = mkOption {
       default = {};
       description = ''
         A set of environment variables used in the global environment.
···
        ''
          ${concatStringsSep "\n" (
            (mapAttrsToList (n: v: ''${n}="${concatStringsSep ":" v}"'')
-             (zipAttrsWith (const concatLists) ([ (mapAttrs (n: v: [ v ]) cfg.systemVariables) ]))))}
+             (zipAttrsWith (const concatLists) ([ (mapAttrs (n: v: [ v ]) cfg.sessionVariables) ]))))}
        '';
 
   };

nixos/modules/config/timezone.nix

···
 
   config = {
 
-    environment.systemVariables.TZDIR = "/etc/zoneinfo";
+    environment.sessionVariables.TZDIR = "/etc/zoneinfo";
 
     systemd.globalEnvironment.TZDIR = tzdir;
 

nixos/modules/programs/environment.nix

···
         EDITOR = "nano";
       };
 
-    environment.systemVariables =
+    environment.sessionVariables =
       { NIX_PATH =
           [ "/nix/var/nix/profiles/per-user/root/channels/nixos"
             "nixpkgs=/etc/nixos/nixpkgs"

nixos/modules/security/ca.nix

···
         }
       ];
 
-    environment.systemVariables =
+    environment.sessionVariables =
       { OPENSSL_X509_CERT_FILE = "/etc/ssl/certs/ca-bundle.crt";
         CURL_CA_BUNDLE         = "/etc/ssl/certs/ca-bundle.crt";
         GIT_SSL_CAINFO         = "/etc/ssl/certs/ca-bundle.crt";

nixos/modules/services/misc/nix-daemon.nix

···
       };
 
     # Set up the environment variables for running Nix.
-    environment.systemVariables = cfg.envVars;
+    environment.sessionVariables = cfg.envVars;
 
     environment.extraInit =
       ''