Restructure systemd services definition. Add environment variables

nixos-config/home-manager.nix

@@ -54,7 +54,10 @@ in
       ./home-manager/fish.nix
     ];
 
-    systemd.user.services = {
+    systemd.user = {
+      # user services
+      services = {
+        # ssh-agent user service
         ssh-agent = {
           Unit = {
             Description = "SSH key agent";
@@ -69,6 +72,11 @@ in
           };
         };
       };
+      # user environment variables
+      sessionVariables = {
+        SSH_AUTH_SOCK = "/run/user/1000/ssh-agent.socket";
+      };
+    };
 
   services.mako.enable = true;
   };