chore(nix): exec 'use flake' with direnv on NixOS systems

.envrc

@@ -2,6 +2,8 @@
 
 dotenv_if_exists
 
-# use flake ".#${DIRENV_DEVSHELL:-default}"
+if [ -f /etc/os-release ] && grep -q '^ID=nixos' /etc/os-release; then
+    use flake ".#${DIRENV_DEVSHELL:-default}"
+fi
 
 PATH_add bin