home: set CARGO_TARGET_DIR to ~/.target/

hosts/home.nix

@@ -80,6 +80,9 @@ in
   home = {
     username = "${user}";
     homeDirectory = "/home/${user}";
+    sessionVariables = {
+      CARGO_TARGET_DIR = "$HOME/.target/";
+    };
 
     stateVersion = "22.05";
   };